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      Foot & Ankle International
      April 1, 2004
      277 Renew Paid Title: Challenges in Total Ankle Arthroplasty - Lowell 
H. Gill, M.D.   (Receive without keywords highlighted)
      Summary: (view full summary) ABSTRACT - In the past, total ankle 
arthroplasty was largely abandoned due to poor survivorship most often 
caused by loss of bone support. High complication rates were also reported. 
Despite this...


Challenges in Total Ankle Arthroplasty

Lowell H. Gill, M.D. Charlotte, NC

ABSTRACT

In the past, total ankle arthroplasty was largely abandoned due to poor 
survivorship most often caused by loss of bone support. High complication 
rates were also reported. Despite this, there is renewed interest in ankle 
arthroplasty and encouraging results are seen in survivorship with midterm 
follow-up. The procedure, however, remains more challenging than total hip 
or total knee arthroplasty. With the limited soft tissue envelope, wound 
problems are not uncommon. Forces at the ankle are very large and yet the 
surface area for prosthetic support is small. Therefore, fixation can be 
more difficult. The strongest bone can be eccentric at the distal tibia. The 
tibial prosthesis can, therefore, tend to settle into the softer bone often 
laterally. Polyethylene needs to be sufficiently thick to maintain its 
integrity but that requires a larger bone resection, which weakens bone 
support. Polyethylene failure or wear leads to the majority of failures in 
hip and knee arthroplasty. There is a need for further basic science 
research in total ankle arthroplasty. The lessons learned from other 
arthroplasty should be considered in ankle arthroplasty design.

Key Words: Agility; Angiosomes; Ankle Arthroplasty; Biomimetic Coatings; 
Bone Support; Buechel-Pappas; Polyethylene; STAR Ankle Arthroplasty

Corresponding Author:
Lowell H. Gill Gill Orthopaedic Clinic Midtown Medical Plaza 1918 Randolph 
Road, Suite 700 Charlotte, NC 28207
E-mail: dvenne на gillortho.com
For information on prices and availability of reprints call 410-494-4994 
X226.

HISTORY

The earliest reports of total ankle arthroplasty were favorable. Stauffer70 
at the Mayo Clinic reported on 63 total ankles with an average follow-up of 
6 months. Of these 63 ankles, 52 were rated excellent, six fair, and five 
poor. In a smaller series with longer follow-up, Lachiewicz et al.46 
reported on 15 total ankle arthroplasties at 39 months postoperatively. All 
results were excellent or good. Other early series similarly reported 
encouraging results.38,59,79

With longer follow-up, however, the reports became more 
cautious.21,29,30,66,76 The terminology used to report results changed. The 
word ''excellent'' became rarely used, and instead series often substituted 
the words ''success'' or ''satisfactory.''76 At times, this only meant that 
the prostheses were still in place.

In time, virtually all series reported larger numbers of 
failures.35,39 -41,52,60,85 Ultimately, almost all authors abandoned or 
largely abandoned total ankle arthroplasty due to the high failure 
rate.29,35,38,40,41,52,60,85 Bolton-Maggs and associates,11 reporting on 62 
total ankle arthroplasties with the ICLH prosthesis, recommended against 
total ankle arthroplasty. They noted, ''in view of the high complication 
rate and generally poor long-term clinical results, we recommend arthrodesis 
as the treatment of choice for the painful stiff arthritic ankle, regardless 
of the underlying pathologic process.'' Years earlier, this same practice 
had reported that their study ''encouraged optimism'' regarding total ankle 
arthroplasty.38 Newton, another early proponent of total ankle arthroplasty, 
subsequently also reported fusion as the procedure of choice.60

Design variations seemed to make little difference. Several authors 
recommended against constrained designs because of a high failure 
rate.39,40,85 However, nonconstrained designs failed as well.41,60

Authors who previously performed arthroplasty recommended arthrodesis, which 
was felt to give more predictable results with fewer 
complications.11,35,41,52,60 Schaap and associates67 reported favorable 
long-term results with an average of 10 years in patients treated with 
arthrodesis. There are additional studies which also show favorable 
long-term results with arthrodesis.54,55 Some authors reported superior gait 
patterns in the arthrodesis patients, whereas more abnormal kinematics and 
marked muscle weakness were documented following total ankle arthroplasty.22 
Mazur and associates55 found all patients had favorable gait studies after 
ankle arthrodesis.

Lord,51 a French surgeon who performed the first total ankle in 1970, 
reported disturbances in balance occurring in total ankle arthroplasty 
patients. These balance abnormalities did not exist in total hip 
arthroplasty patients and were much milder in total knee arthroplasty 
patients.51 There was also noted decreased anteroposterior stability 
following laboratory total ankle arthroplasties using a meniscal bearing 
with a flat upper surface.16 Many years later using the Scandinavian Total 
Ankle Replacement (STAR) prosthesis, which employs such a design, Garde and 
Kofoed26 reported satisfactory stabilometry studies following total ankle 
arthroplasty.

Summarizing the early experience, it should be noted that initially total 
ankle arthroplasty was successful. However, these procedures were ultimately 
abandoned because of the high failure rates. Today's surgeons should 
therefore still use caution in the optimism with the present designs, which 
also appear favorable in the early and midterm reports.

COMPLICATIONS AND WOUND HEALING

Total ankle arthroplasty is associated with a high complication 
rate.11,21,22,29,30,35,71 The procedure is technically challenging. There is 
a risk of fracture of one or both malleoli. Neurovascular structures are in 
close proximity. Laceration of the posteromedial tendons from saw cuts using 
the anterior approach can occur. Wound healing problems are not 
unusual.29,65 The vascular supply may be more likely compromised by arterial 
disease at the level of the ankle. As a result of more constricted 
tethering, the vascular supply of the ankle does not tolerate the 
dislocation28 that is done for total hip arthroplasty nor the marked 
subluxation that is performed at the time of total knee arthroplasty. The 
soft tissue envelope is sparse at the ankle and has minimal flexibility.3 At 
the subcutaneous surface of the tibia where deep fascia is continuous with 
the periosteum, branches of the anterior tibial artery which supply the skin 
are easily torn by shear forces.74 The dorsalis pedis is absent or extremely 
attenuated in 12% of cases,3 and this is the main arterial supply to the 
dorsum of the foot.

ANGIOSOMES

An angiosome is a block or three-dimensional area of tissue supplied by a 
specific source artery. The angio-some may include bone, muscle, fascia, 
subcutaneous tissue, and skin. In many areas of the body, such as the 
forearm, there are rich intramuscular anastomoses between different 
angiosomes.74 Four of the five angiosome areas of the leg have blood supply 
from more than one angiosome. The angiosome supplied by the anterior tibial 
artery, however, has circulation supported by only one source artery, the 
anterior tibial artery.74 For this reason the anterior compartment leg 
muscles are particularly vulnerable to ischemia. After a vascular insult to 
the source artery of an angiosome, it is possible for closed or reduced 
caliber connections termed ''chokers'' to open and supply the structures of 
an adjacent angiosome. However, this process can take 3-10 days, which 
places the structures in an angiosome at risk for necrosis when there is 
only one supply.3,4 The safest incision in the foot and ankle is at the 
junction of two angiosomes.4 In this way both sides of the incision are 
likely to have healthy and independent blood supply. A lateral approach has 
this advantage. The anterior approach to the ankle, however, divides a 
single angiosome approximately in the middle. The anterior incision is the 
one most commonly used for total ankle arthroplasty. This incision is in the 
angiosome supplied by the anterior tibial artery and its continuation as the 
dorsalis pedis. The proximal part of the incision may lie in the anterior 
compartment of the leg where there is greater risk of ischemia. More 
distally at the level of the ankle and foot there are anastamoses to other 
vessels, but at this more distal level there are other risks previously 
outlined.

Either an anterolateral or an anteromedial incision can potentially be at 
risk. For example, if the neurovascular bundle is retracted laterally, then 
the two medial anastomoses from the anterior tibial to the posterior tibial 
vessel are likely ligated or injured. In this situation if the lateral 
peroneal anastomoses are vestigial or blocked, then healing is at 
considerable risk. On the other hand, if the surgeon approaches 
anterolaterally and retracts the vascular bundle medially, then the lateral 
anastomoses are likely interrupted. In this situation if the medial 
anastomoses from the posterior tibial artery are ineffective, then again the 
anterior incision is at considerable risk.

Summarizing, the anterior angiosome itself has only a single arterial source 
in the leg. The midline anterior approach is less desirable than the border 
areas between angiosomes. The anastomoses that do exist are at risk and 
easily injured. Vessel anomaly is common. A suggested plan for the surgeon 
is to use a doppler preoperatively to map out individual precise 
circulations.

SUPPORT

The most frequent complication of total ankle arthroplasty in the past has 
been loss of bone support.



Fig 1: Talar component subsidence.

Most orthopaedic prostheses depend primarily on bone for support. 
Unfortunately, however, many patients needing prosthetic arthroplasties have 
weakened or compromised bone. Past experience with total ankle arthroplasty 
has shown that loss of support is a primary reason for failure (Fig. 1).

Bone Strength

The importance of bone support was recognized early in total ankle 
arthroplasty and attention has focused on the increased risks in patients 
with bone depleted by osteonecrosis, long-term disease, chronic inactivity, 
or steroid use.59 An early laboratory study38 looked at total ankle 
arthroplasty support by performing total ankle arthroplasties in cadavers 
and subjecting these arthroplasties to physiologic forces. The study found 
failure of the support bone around the prostheses in just a few days. 
Studies of three-dimensional models of talar and tibial components of 
implanted ankle prostheses have shown that by removing the cortical shell of 
the talus, abnormally increased stresses are placed on the remaining talar 
bone.17 Bone strength at the ankle has been studied and there is marked 
reduction in the bone strength as the sections are taken farther from the 
articular surface. The talar bone was found to be 40% stronger than the 
distal tibial bone, which was noted to be dangerously close to or below the 
failure point for prosthetic replacement at the ankle (Fig. 2). Distal 
tibial bone strength should equal or exceed 20 MPa.33 The strongest bone is 
not central nor evenly distributed across the distal tibia, but is in fact 
eccentric, usually posteromedial33 (Fig. 3). Since maximum bone strength is 
eccentric, and strongest in a specific small area reflecting the 
transmission of the force of heel strike, this can produce a type of pivot 
point which could lead to tibial component subsidence into the weaker 
surrounding bone, which is usually anterolateral (Fig. 4).



Fig. 2: Minimal amount of strong bone at distal tibia. Arthroplasty 
resection removes best support bone.



Fig. 3: Minimal Area of maximal bone strength is often eccentric.

Force

Forces at the lower extremity are large due to the principle of leverage, 
which magnifies the force of body weight. Lower extremity forces are 
particularly increased at the ankle.2,69-72 Since the forefoot metatarsal 
pad is a greater distance from the fulcrum at the ankle joint compared to 
the shorter distance from the ankle to hindfoot, this creates a longer 
anterior lever arm at the foot. During ambulation, therefore, the Achilles 
tendon must generate very large tensile forces to overcome the body weight 
on the longer lever arm of the forefoot. This results in very high 
compressive forces at the ankle.

Ankle compressive forces are estimated to be three to five times body weight 
during normal walking.22,69,72 In one study,22 marked muscle weakness was 
documented in ankle arthroplasty patients. Due to their muscle weakness, the 
total ankle arthroplasty patients did not or were not able to generate a 
normal compressive load at the ankle. This may be good for prosthesis 
survival, but not advantageous for ambulation. It should



Fig. 4: Eccentric bone support potentially causes uneven prosthetic 
subsidence.

be noted that the forces at the ankle are large, yet laboratory studies have 
shown that bone strength is often compromised at this same location.

Surface Area

As the force across the ankle joint cannot be markedly influenced by a 
prosthetic design, the surface area contact between the prosthetic component 
and resected bone becomes critical for success. Forces are commonly measured 
in Newtons. One Newton equals the force required to lift 1 kg of mass 
against gravity kg-m/s2). What is critical in prosthetic design is the 
pressure applied by the prosthesis to the bone. Pressure a measure of the 
force per unit area. A Pascal (Pa) equal to 1 N spread over 1 m2 (N/m2). The 
strength bone is measured in the same units as pressure (Pascals). Thus, as 
the surface area is increased, the pressure is decreased, and vice versa.

Early total knees were available in only one size. Often the tibial 
component was prone to subsidence (Fig. 5A). Today's tibial components are 
available in multiple sizes allowing better prosthetic support through the 
expansion of support surface area (Fig. 5B).

The actual surface area of the ankle joint is 12 cm,2 which is large 
compared to the hip or knee.70 Much this surface area is in the medial and 
lateral gutters and on the relatively large anteroposterior dome of the 
talus. Depending on the particular design, much of this surface area may not 
be available for prosthetic support. The talus is a small bone. When the 
dome of the talus resected, this results in approximately one half the 
surface area as that of the upper tibia at the knee. The compressive force 
at the knee is three to four times body weight on a larger surface area, 
whereas at the ankle during ambulation there are compressive forces



Fig. 5: A, Subsidence of a single-sized tibial total knee component with 
inadequate base plate coverage. B, Newer base plates improve bone coverage 
for better support.



Fig. 6: Smaller surface area for support at ankle.

of up to 5.5 times body weight on a much smaller surface area. This greatly 
increases the load per unit area (Fig. 6).

The addition of a keel expands surface area, reduces force per unit area, 
and greatly reduces micromotion.24, 78 The small size of the talus allows 
little room for a keel if one is to preserve sufficient support bone. The 
proximity of the subtalar joint completely prevents the expansion of the 
keel distally and the confines of the narrow talus prevent expansion of the 
keel medially and laterally.

The force borne across the ankle is often not central nor equally placed 
across the prosthetic support surfaces (Fig. 7). Instead the force is often 
off-center (i.e., eccentric). The eccentric force across the pros-thesis 
leads to a compressive or intrusive force on one side and an elevation or 
lift-off force on the opposite side.36, 77 (Figs. 4 and 7). Shear forces 
also result which increase the stress in the underlying cancellous bone.7 
Studies in cadaver tibial knee arthroplasties showed that four peripheral 
screws with a central peg best resists the micromotion of the tibial base 
plates which



Fig. 7: Stresses across prosthetic ankle may be eccentric.

results from eccentric force.77 Another study which included a keel in the 
selection of base plate designs found that a keel consistently best resists 
eccentric and shear forces. The worst design of the five designs tested was 
the tibial base plate with no understructure.24 At the ankle because of the 
anatomic limitations of the talus, it may be impossible to provide either 
four screws plus a central stem or a keel.

In summary, it has already been stated that bone strength at the ankle is 
not evenly distributed but maximal strength is instead eccentric (Fig. 3). 
Forces that result from normal human activity are also often eccentric. Any 
malalignment (Fig. 8) may aggravate the eccentric distribution of force in 
the bone, which is not evenly strong. The eccentricities may not match. At 
the ankle there is minimal surface area available for the distribution of 
force and it has already been documented that bone strength is often 
marginal if not even inadequate. A total ankle arthroplasty is



Fig. 8: Malalignment potentially aggravates eccentric force and resultant 
subsidence.

therefore always at risk for failure because of inadequate bone support.

MATERIALS: POLYETHYLENE

Initially polyethylene was thought to be a nearly ideal material for 
arthroplasty. It provides low friction when articulating with metal in vivo. 
Earlier studies suggested the amount of wear was acceptable and the wear 
particles were thought to be innocuous.38 Wear studies suggested minimal 
wear allowing longevity of 20 years or longer with the available designs.

Clinical observation has proved many of the above assumptions as false and 
the early laboratory studies as misleading.10,61,68 There are numerous 
different patterns of wear and the causes of failure are 
multifactorial49,50,68 (Fig. 9). The magnitude of the polyethylene problem 
is seen clearly in a US report of medical device failures.20 It is estimated 
that only 1-5% of such failures are actually reported. A study of 1,717 
total hip and 2,769 total knee arthroplasty failures that were reported 
documents the significance of the polyethylene problem. Polyethylene failure 
was the most common cause of total hip failures and accounted for 68% of 
total knee failures.20 Early laboratory wear studies often utilized 
pin-on-disk or linear track motion, both of which provided misleading and 
overly optimistic predictions.49, 50 Poly-ethylene wear is reduced with 
these types of motion in the laboratory. Clinically, however, the 
crossing-path type of motion, which occurs in vivo, produces greater wear. 
Retrieval studies document the severity of the wear (Fig. 9).

We now know that particulate polyethylene debris may cause osteolysis32, 37 
(Fig. 10). Polyethylene particles in sufficient numbers incite a chronic 
inflamma-tory process which leads to osteolysis.37 Particles of small size 
(less than 15 чm) are phagocytized by



Fig. 9: Complete wear-through of a tibial polyethylene component.



Fig. 10: Large area of osteolysis caused by polyethylene wear in less than 7 
years.

macrophages. In response to the phagocytosis of these small sized particles, 
a cascade of events occurs and the end result is osteolysis. As this 
progresses, the osteolysis leads to aseptic loosening and eventual loss of 
support.32 The yield strength of polyethylene is relatively low, between 13 
and 25 MPa.10, 84 The developers of the Buechel-Pappas (BP) ankle (Endotech, 
Inc., S. Orange, NJ) present data showing computed surface contact stresses 
for the BP ankle on polyethylene to be less than 5 MPa, which is well below 
the yield strength of polyethylene.15 This same report notes contact 
stresses on polyethylene to be 32 MPa for a fixed polyethylene two-component 
design.15 The lessons learned during the time of round-on-flat polyethylene 
total knee designs have shown extremely high failure rates in the past in 
part due to excessively high contact stresses on the polyethylene. These 
observations in knee design should be considered in total ankle arthroplasty 
design.

Thin polyethylene wears faster than thick polyethyl-ene.6, 7 It is estimated 
that a minimum of 4-6 mm of polyethylene thickness is needed at the hip and 
6-8 mm at the knee where there are larger forces and less conformity. 
Optimal thickness at the ankle has not been determined.

Metal backing of polyethylene improves force distribution to the nearby 
cancellous bone and allows in-growth but also requires another 2 mm of bone 
resection, or decreases the thickness of polyethylene. The metal backing, 
particularly if there is a lack of polishing, causes backside wear of the 
polyethylene. Backside wear can be severe in both hips and knees. This 
underscores the importance of the newer and improved locking mechanisms for 
polyethylene.

Polyethylene osteolysis was not reported in the early series of total ankle 
arthroplasty. Early total ankle arthroplasties probably did not last long 
enough for polyethylene failure to become manifest. Also polyethylene 
osteolysis was not widely recognized until after the development of 
cementless fixation. Prior to that time osteolysis was usually thought to be 
secondary to cement (i.e., ''cement disease''). We now know that cement 
disease is actually particle disease and that particulate debris from a 
variety of different materials, including polyethylene, can contribute to 
bone loss.32 Present design total ankle arthroplasties now show improved 
survival rates and therefore polyethylene problems may become more apparent 
in total ankle arthroplasty.

Fracture of the mobile polyethylene component had been reported in separate 
series of STAR (Waldemar Link GmbH & Co., Hamburg, Germany) 
arthroplasties.19,43,80 The typical history is a sudden catastrophic event 
followed by pain and swelling in the involved ankle.43 This has occurred 
rarely and most commonly in physically active people such as hikers.43 The 
phenomenon of edge loading on the polyethylene component of total ankle 
arthroplasties has been reported.80,81,83 This causes excessive wear and is 
described in a recent review of 200 STAR ankle arthroplasties.83 
Polyethylene failures have also been reported due to excessive wear in the 
BP total ankle.15 Osteolysis has been reported with both the Agility (DePuy, 
Inc., Warsaw, IN) and STAR prostheses.62,80,83 Although most reports are of 
radiographic findings, the presumptive etiology is polyethylene osteolysis 
as is commonly seen in hip and knee arthroplasty.

The failures of polyethylene have led to the search for improved 
polyethylenes as well as for alternative bearing surfaces. Past attempts to 
improve polyethylene include the development of Poly II and Hyalmer.49 Poly 
II included a composit of carbon fibers which were added to reduce creep 
(cold flow) of polyethylene. Hyalmer is polyethylene with altered polymer 
morphology. In clinical usage, however, both ''improvements'' failed in the 
sense that their performance was inferior. These products have been 
discontinued for total joint usage. Although laboratory tests suggest 
greatly improved wear characteristics with the newer highly cross-linked 
polyethylenes, the effects of this process on fatigue and fracture 
resistance properties of the polyethylene are not yet known.49 It is 
important to remember that past attempts to improve polyethylene have failed 
to provide superior performance in vivo.50

FIXATION

The early total ankle arthroplasties used polymethyl-methacrylate cement for 
fixation. This fixation was often lost, however, when the bone support 
failed, which was the most common mode of failure. Virtually all current 
ankle arthroplasty designs employ cementless fixation which potentially 
offers a more permanent long-term bond provided the bone support is not 
lost.

Astudyofcementedstainlesssteelmetalandpolyethy-lene total ankle 
arthroplasties compared with unce-mented ceramic-on-polyethylene total ankle 
arthroplasties recommended the cementless technique.73 Since the cementless 
ankles were an average of only 4.1 years postoperative, whereas the cemented 
ones averaged 8.1 years postoperative, meaningful conclusions regarding the 
use of cement or cementless technique are not clari-fied in this comparative 
study.

There is a paucity of laboratory study on cementless fixation in ankle 
arthroplasty. However, recent clinical series which use cementless fixation 
report successful midterm survivorship.1,13,15,43,44,62,80,81-83 Those 
results are improved compared to earlier series.11,29,35,38-52,60,85

Cementless fixation occurs with on-growth onto the surface of a prosthetic 
component or in-growth into a roughened coating applied to the surface of a 
prosthesis. In-growth can occur into a roughened surface such as that 
obtained with sintered beads, plasma spray metals, or fiber metals. These 
roughened microsurface treatments are added as an external layer on to the 
surface of the prosthesis.

Osteoconductive coatings may be added also in order to stimulate bone growth 
at the bone-prosthesis interface. Calcium phosphate ceramics such as 
hydrox-yapatite can be applied to the prosthetic surface with a plasma spray 
technique. This technique as a line-of-sight process tends to coat the high 
spots on the outside of the roughened coating and misses the inner surface 
of a three-dimensional microstructure coating.

The newer biomimetic coating techniques involve a precipitation in a 
supersaturated Ca (PO4)2 solution done at low temperatures. As an immersion 
technique this has the ability to coat more fully the inner geometry of a 
three-dimensional microstructure surface coating applied to a prosthesis.27 
The potential benefits of bioactive coatings are the improved strength of 
bone prosthetic bonding, an accelerated response of the bone at the implant 
junction, improved filling of gaps, and the elimination of the fibrous layer 
that can occur between the prosthesis or cement and bone.

Both laboratory and clinical investigations support the use of 
hydroxyapatite18,75 but the success can vary according to the specific 
prosthetic component treated,27,47,57,63 the specific area of use, and 
whether or not there is a roughened surface treatment.9 For example, 
hydroxyapatite added to a smooth femoral hip arthroplasty component has 
shown long-term success, whereas this same treatment on a smooth acetabular 
component has shown a much higher failure rate.9,47,57,63 This is another 
example, beside that of polyethylene, of a material transfer phenomenon 
where a material may not behave in the same fashion when transferred to a 
different area. Therefore, success at the ankle would not be necessarily 
assumed simply because of the success on the femoral components of total hip 
arthroplasties.

In the United States, ankle prosthetic components are sold without bioactive 
coatings. The Agility total ankle arthroplasty has a porous-coated cobalt 
chrome surface. The BP has a beaded titanium surface for in-growth. The STAR 
prosthesis in current use in the United States is a titanium porous coating 
on a cobalt chrome prosthesis without hydroxyapatite or calcium phosphate. 
This prosthesis is made available to selected surgeons who are part of a 
multicentered study.

Bioactive coatings have been added to total ankle arthroplasty components in 
Europe and Japan. The TNK prosthesis (TNK ankle, Nara, Japan) has a ceramic 
component coated with hydroxyapatite. Two ankle designs similar to the BP 
sold in Europe, the Alpha-norma OSG ankle (Corin Group Co., Quierschied, 
Germany) and the AES (Ankle Evolution System) (Biomet Merck Valence, Cedex, 
France) ankle have a double coating surface which includes hydroxyapatite. 
The HINTEGRA ankle (New Deal Co., Vienne, France) has a double-coated porous 
titanium and hydroxyapatite surface. In the year 2000, the STAR prosthesis 
was made available in Europe using a dual coating of calcium phosphate which 
is electrochemically bonded onto a titanium porous coating which is applied 
to the cobalt chrome prosthesis. The advantage of the electrochemical 
application of calcium phosphate is that this process allows better 
distribution of the bioactive surface throughout the interstices of the 
microstructure of the titanium coating since it is an immersion process. The 
above ankles sold in Europe are examples of the ''second line of defense'' 
concept in surface treatment.

In a review of 200 cementless STAR total ankle arthroplasties, Wood noted 
significantly improved radiologic appearance in the newer dual-coated STAR 
prostheses compared with the earlier hydroxyapatite-coated cobalt chrome 
prostheses.81,83 Similarly, Bonnin12 reported improved radiologic appearance 
on the bioactive coated Salto Total Ankle prosthesis compared to earlier 
Salto ankle arthroplasties without the bioactive coating.12

DESIGN

In speaking of total knee design, John Insall stated that knee arthroplasty 
design was based more on opinion than scientific study.34 The same may be 
true for ankle arthroplasty. There have been comparatively few laboratory 
studies on the design criteria for total ankle arthroplasty. Falsig and 
associates25 looked at stress transfer to distal tibial trabecular bone with 
three different generic tibial designs at the ankle as follows: (1) a 
polyethylene tibial component, (2) a metal-backed polyethylene component, 
and (3) a long-stem metal-backed tibial component using a much longer stem 
than is common. With these three designs, an eccentric anterolateral load of 
2,100 N (approximately three times body weight) was applied and compressive 
stresses in the bone were measured. The authors found a 25% reduction in 
trabecular bone stress to 15 N/mm2 by adding metal backing to the 
polyethylene component. Shear stresses were also reduced. The addition of 
the long stem, however, resulted in almost complete reduction of trabecular 
bone stress in the distal tibial bone since most stress was transferred to 
the long stem. The authors postulated that this situation may lead to 
excessive stress shielding in the distal tibial bone and therefore could 
adversely affect a long-term clinical result.

Based on the few available laboratory studies looking 
atbonestrengthattheankle33 andtotalanklearthroplasty studies17,48 as well as 
the information available from hip and knee arthroplasty, it appears that 
goals for total ankle arthroplasty may be as outlined in Table 1.

Review of these goals show that some are difficult to achieve or even 
contradictory. Achieving goal 4 (i.e., use thicker polyethylene), for 
example, directly inhibits the ability to achieve goal 1, which is to 
minimize bone removal. Furthermore because of the small size of the distal 
tibia and talus, goals 2 and 3 (maximizing surface area for support and 
stabilization) are very difficult to achieve.

Designs vary considerably in the amount of bone area resurfaced in total 
ankle arthroplasty. Although data are not available providing guidance on 
how much area at the ankle should be resurfaced, from a force distribution 
standpoint it is desirable to maximize the area for resurfacing. On the 
talar side, the STAR maximizes the area of resurfacing by including the



medial and lateral talar facets in addition to preserving part of the dome 
of the talus. Theoretically this may improve force distribution and 
long-term stability of the talar component.

It has not been determined, however, if it is in fact necessary to resurface 
the medial and lateral facets. The BP ankle is an on-lay component of the 
superior surface of the talus only with two fins in the talar dome. By not 
resurfacingthemedialandlateraltalarfacets,lesscortical bone is removed from 
the talus. Saltzman points out that with each additional area resurfaced 
greater operative exposure and more bone removal are required.65 Without 
resurfacing the medial and lateral talar facets, there is a theoretical 
concern of persistent postoperative pain from the nonresurfaced facets. 
However, surgeons experienced in both the STAR and BP total ankles report 
that medial and lateral facet pain has not been a clinical problem with the 
BP ankle.64,81 Rippstein has found that it is not necessary to resurface the 
facets.64 With regard to resurfacing of the facets, the trade-off therefore 
is the potential benefit of increased surface area for stability and 
fixation by including facet resurfacing versus the potential benefit of 
preservation of the strong medial and lateral cortical bone by not 
resurfacing these areas.

Kinematics

Arthroplasty alters normal kinematics at the ankle. Rather than being a 
simple hinge joint, Michelson et al.56 found that the ankle moves ''as a 
complex joint with coupled three-dimensional motions.'' The talus is wedge 
shaped with different radii of curvature on the medial and lateral talar 
domes as well as different radii of curvature anteriorly and posteriorly.5 
Therefore, the ankle joint axis changes continuously throughout the range of 
motion.53 The axis of motion can vary considerably and may vary among 
different individuals.5,53

With the exception of the HINTEGRA, most current ankle arthroplasty designs 
do not employ a different radius of curvature on the medial and lateral 
aspects of the talus. In the normal anatomy, there is a slightly smaller 
curvature medially. Theoretically, an arthroplasty with symmetric equal 
curvatures on the medial and lateral aspects of the talar component could 
result in a ligamentous imbalance which is tight medially and loose 
laterally. In arthroplasty designs with a mobile bearing, the flat geometry 
on the upper side does not reproduce the convex-concave articulation of the 
talus in the tibial mortis. The normal anatomy, therefore, has more inherent 
anteroposterior stability. Theoretically, the lack of the convex-concave 
shape in the sagittal plane puts more stress on the ankle ligaments. Proper 
ligamentous balance and stability therefore may be even more important 
following prosthetic replacement than in the normal ankle, especially in a 
relatively unconstrained prostheses such as the STAR, BP, and HINTEGRA. 
Despite the potential advantage of a more physiologic tensioning of ankle 
ligaments with a truncated talar component, the BP and STAR arthroplasties 
appear to work well in the hands of experienced surgeons.15,45,83

Bearing Surfaces: Fixed vs. Mobile Bearings

Present total ankle arthroplasty designs use a polyethylene-bearing surface. 
The Agility polyethylene measures from 3.73 mm to 4.7 mm and additional plus 
2-mm inserts are available.23 Other popular designs also have relatively 
thin polyethylene when compared to total knee arthroplasty in which 6-8 mm 
is recommended. Since bone cuts must be kept conservative, there is not 
sufficient room remaining to allow two metal components that are a minimum 
of 2-3 mm in thickness each and still allow sufficiently thick polyethylene. 
A fixed polyethylene-bearing surface may potentially reduce backside wear if 
there is an effective locking mechanism. The Agility ankle and the Eska 
developed in Germany use a fixed bearing.

A mobile bearing by definition allows backside wear but may be made fully 
conforming, which greatly reduces contact stress in the polyethylene. Most 
newer design total ankle arthroplasties use a mobile bearing. In the United 
States, mobile bearings are used for the STAR and the BP ankles. In Europe, 
in addition to the STAR and BP (Wright Cremascoli Ortho S.A., Toulon-Cedex, 
France), the HINTEGRA, the AES, the Salto, and the Alpha-norma OSG ankle all 
use a mobile bearing. An advantage of the mobile bearing concept is that the 
flat upper surface allows some rotation which reduces stress at the 
prosthesis-bone interface. A potential disadvantage is that the flat 
geometry does not reproduce the convex-concave articulation of the talus in 
the tibial mortis. Studies that look at ankle stability after prosthetic 
replacement show conflicting results, although some studies document 
increased instability.16,26,51 The BP ankle design may allow better contact 
at the bearing surface because of its curving geometry under adverse loading 
conditions, such as tilting due to malalignment or ligament imbalance.81 
Even the mobile bearing STAR design may be prone to edge-loading.81 Fixed 
two-component designs can be prone the problem of edge-loading especially if 
there is any malalignment. Edge-loading will increase contact stresses in 
the polyethylene.

A review of the three ankle arthroplasty designs in current use in the 
United States follows.

Agility1 (Fig. 11)

  a.. The Agility ankle employs a unique feature of an arthrodesis of the 
distal fibula to the distal tibia at the time of surgery. This expands the 
surface area available for support on the tibial side by utilizing the 
distal fibula for additional support. A nonunion of this important 
arthrodesis, however, risks loss of fixation on the upper side.


Fig. 11: Agility ankle prosthesis. The upeer cononent includes a 
polyethylene insert.

  a.. The polyethylene insert into the metal-backed tibial component is 
concave in the sagittal plane. This adds anteroposterior stability.
  b.. A deliberate mismatch exists between the larger upper tibial component 
and the smaller lower talar component. This mismatch allows the talus to 
seek its own position and allows freedom from excessive constraint 
protecting bone-prosthesis interfaces.
  c.. The deliberate mismatch in sizing of components could potentially 
allow increased contact stresses in the polyethylene if any malalignment or 
ligament imbalance led to ''edge-loading'' of the talar component.
  d.. The polyethylene component is relatively thin. It does not have the 
expanded surface area for fixation of polyethylene as used in the newer 
locking mechanisms. The locking mechanism relies on a medial and lateral peg 
only as well as a posterior stop as opposed to a circumferential or multiple 
fixation point locking mechanism. Without any anterior capture it does not 
circumferentially capture the polyethylene as in many of the newer locking 
mechanisms.
  e.. The prosthesis resurfaces the tibiotalar surface as well as the medial 
and lateral facet areas.
  f.. The talar component requires a relatively aggressive talar cut leaving 
less talar bone available for support.
  g.. The early talar design did not take advantage of the entire available 
surface area for support. A modified newer version partially improves this 
situation.
  h.. Insertion of the entire prosthesis requires relatively aggressive bone 
cuts. A distracter used at the time of surgery helps reduce this problem but 
the amount of bone removal is still substantial.
Buechel-Pappas Total Ankle8,9 (Fig. 12)

  a.. BP total ankle is a three-component design, which utilizes a mobile 
polyethylene bearing.


Fig. 12: Anterior A and lateral B, views of Buechel-Pappas total ankle 
prosthesis.

  a.. The mobile bearing reduces excessive stress transfer to the 
bone-prosthesis interface.
  b.. There is full conformity between the polyethylene component and the 
tibial and talar components.
  c.. The prosthesis resurfaces only the tibiotalar area and not the facets.
  d.. Because the bearing is mobile, there is automatically backside wear.
  e.. The tibial component has a short stem. This may potentially protect 
tibial trabecular bone but avoid the excessive stress shielding from an 
overly long stem.
  f.. The talar component is an on-lay component with two fins for fixation. 
It preserves most of the talar dome. Since it does not resurface the medial 
and lateral talar facets it thereby helps preserve talar cortical bone.
  g.. The flat upper surface of the mobile bearing may reduce 
anteroposterior stability.
Star30, 31 (Fig. 13)

  a.. This prosthesis also has a mobile bearing polyethy-lene component.
  b.. The prosthesis resurfaces the tibiotalar articulation and provides a 
hemi-resurfacing of the two facet areas.
  c.. There are two dowels for tibial component fixation. This presents a 
lower surface area for stress distribution in the distal tibia compared to 
the BP ankle but might also reduce stress shielding from a stem.


Fig. 13: The Scandinavian Total Ankle Replacement (STAR) prosthesis.

  a.. Talar bone cuts remove less bone than is commonly removed with the 
Agility ankle.
  b.. Talar fixation is enhanced by medial and lateral resurfacing which 
expands the surface area for support on the lower side. Therefore, surface 
area for fixation and load distribution is maximized on the talar component. 
The removal of medial and lateral facets decreases the amount of remaining 
cortical bone.
  c.. The flat upper surface of the mobile bearing may reduce 
anteroposterior stability.10
All three ankle arthroplasties have shown acceptable short-term and midterm 
results in clinical trials.1,13 -15,19,42,43,45,62,80,81, 83 Longer term 
follow-up is not yet available.

SUMMARY

The stimulus for total ankle arthroplasty derives from a partial 
dissatisfaction with ankle arthrodesis49,58,59 as well as success seen with 
total hip arthroplasty and total knee arthroplasty. Total ankle arthroplasty 
is more challenging than total hip arthroplasty and total knee arthroplasty 
due to the limitations of bone strength, the marked limitation of the 
anatomic size of the talus, and the magnified compressive forces distributed 
across the ankle due to the longer lever arm of the foot. Healing problems 
are also much more common at the ankle. Early total ankle arthroplasties 
were initially successful and reported as ''excellent.'' However, with 
longer follow-up these failed largely due to insufficient bone support.

Bone support at the ankle may be marginal. The strongest bone is often 
eccentric. Forces may also be eccentric causing a compressive force on one 
side of a prosthesis and lift-off force on the contralateral side. 
Malalignment may aggravate eccentric loads on prostheses causing compressive 
forces on weaker underlying bone. Forces are large at the ankle but the 
surface area for support is small. There is little to no room to provide a 
keel in the talus and a keel has been shown to best resist eccentric forces. 
Polyethylene has been the primary cause of arthroplasty failure in the hip 
and knee leading to interest in alternative bearing surfaces. Current ankle 
arthroplasty designs use polyethylene.

Successful design of total ankle arthroplasty has been far more challenging 
than at the hip or knee. There is a paucity of laboratory studies of ankle 
arthroplasty to help guide appropriate design. Laboratory investigation is 
essential and will hopefully improve the long-term success with this 
procedure and prevent another series of failures.

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----- Original Message ----- 
From: "Batal" <orthoforum на weborto.net>
To: <ortho на weborto.net>
Sent: Saturday, May 19, 2007 10:49 PM
Subject: [Ortho] эндопротезирование или артродез


>К нам обратился пациент 73 лет с жалобами на боли и деформацию в
> области левого голеностопного сустава.
> В анамнезе: в 1980 году перелом обеих лодыжек с вывихом стопы кнаружи.
> Лечился консервативно: одномоментная репозиция переломовывиха в левом
> голеностопном суставе с трансартикулярной фиксацией голеностопного
> сустава спицами Киршнера через пяточную, таранную, большеберцовую
> кости. Накладывалась гипсовая иммобилизация. Даная манипуляция
> осложнилась нагноением с развитием гнойного артрита. Были удалены
> спицы, сустав со слов больного, промывался растворами, но не
> вскрывался, получал антибиотики.
>
> Гнойный процесс был купирован, и рецидива с тех пор не было. Об-но:
> Левый голеностопный сустав деформирован за счет разрастания костной
> ткани и отечности мягких тканей. Левая стопа с вальгусной установкой,
> практически отсутствуют все своды левой стопы (стопа плоская). Полный
> объем движений в левом голеностопном суставе максимум достигает 15 гр,
> движения стопы в основном за счет подтарнного сустава. Пальпаторно
> область голеностопного сустава не столь болезненна, как болезненна
> область подтаранного сустава и область таранно-ладьевидного сустава.
> После изучения объективного статуса, анамнеза, рентгенснимков,
> больному предложен был трехсуставной артродез, так-как мы сочли это
> наиболее приемлемым в данном случае. Но больной отказывается от данной
> операции и настаивает на эндопротезировании левого голеностопного
> сустава. Во первых, наше отделение не имеет опыта в эндопротезиовании
> голеностопного сустава. Во вторых, нам кажется, что трехсуставное
> артродезирование в данном случае наиболее подходящее. Причиной тому,
> на наш взгляд, выраженная деформация левой таранной кости, как
> следствие аваскулярного некроза, и то, что болит не голеностопный
> сустав в данном случае (хотя в нем и ограничено движение), а
> подтранный и таранно-ладьевидный суставы, и то, что эндопротезирование
> одного голеностопного сустава не решит проблем в подтаранном,
> ладьевидно-таранном сочленениях. Наши доводы оказались безуспешными, а
> так как пациент  является ученым, требовал доказательной базы наших
> умозаключений. Ваше мнение по данному случаю, и мы были бы благодарны,
> если у кого то есть материал по данной теме или есть ссылки. Заранее
> благодарны всем, кто примет участие в обсуждении данной темы.
> Batal
>


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----------- следущая часть -----------
Вложение не в текстовом формате было извлечено&hellip;
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Вложение не в текстовом формате было извлечено&hellip;
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Вложение не в текстовом формате было извлечено&hellip;
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Вложение не в текстовом формате было извлечено&hellip;
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Вложение не в текстовом формате было извлечено&hellip;
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Вложение не в текстовом формате было извлечено&hellip;
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Вложение не в текстовом формате было извлечено&hellip;
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Вложение не в текстовом формате было извлечено&hellip;
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Вложение не в текстовом формате было извлечено&hellip;
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Вложение не в текстовом формате было извлечено&hellip;
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Вложение не в текстовом формате было извлечено&hellip;
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Вложение не в текстовом формате было извлечено&hellip;
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