51 Mobile-Bearing Unicompartmental Knee Arthroplasty
D. G. Murray
51
Summary
Medial unicompartmental knee arthroplasty (UKA) has many advantages over total arthroplasty (TKA) including better function and less morbidity. However, the long- term failure rates of many designs of fixed-bearing UKA are high because of polyethylene wear, lack of sophisti- cated instrumentation, and imprecise patient selection.
Introduction
The fully congruent mobile bearing of the Oxford UKA exhibits minimal polyethylene wear, and failure from this cause does not occur. The instrumentation allows precise implantation to restore isometric function to the liga- ments. During its 20-year development, the limits of use- fulness of the implant have been established and found to include about one in three knees requiring replacement.
There are now a number of published series, using these selection criteria, which have achieved very high survival rates at 10 and 15 years.
Since 1998, the phase-3 implant has been used with modified instruments through a small incision, avoiding damage to the extensor mechanism. Patients now recov- er substantially faster than after TKA or traditional open UKA and regain better function with no compromise in their survival rate.We believe that minimally invasive Ox- ford UKA is the treatment of choice for medial compart- ment osteoarthritis, provided the correct indications are applied and the appropriate surgical expertise is avail- able.
Advantages
Medial unicompartmental knee arthroplasty (UKA) pre- serves all the undamaged structures of the joint, in par- ticular the cruciate ligaments, and can therefore restore knee function nearly to normal. After UKA the range of movement is better than after TKA, the knee feels more natural, and pain relief is as good or better [1-3]. In terms of morbidity, operative blood loss is less and transfusion
unnecessary; complications are less frequent and less se- rious, and recovery is more rapid. However, UKA has the disadvantage that the medium and long-term revision rates are generally higher than for TKA. The reasons for this include:
1. The high polyethylene wear rate of thin tibial compo- nents subjected to incongruous loading
2. Imprecise (and inappropriate) limits for patient selec- tion
3. Lack of instruments to accurately implant the device
Over the years we have addressed these points and have developed a unicompartmental system that should have a long-term survival similar to that of TKA.
Material
The Oxford UKA has spherical femoral and flat tibial components, both made of cobalt chrome. Between them lies an unconstrained mobile bearing, the upper surface of which is spherically concave and the lower surface flat, so that it is fully congruent with both metal components in all positions.The contact area is large (about 6 cm
2) and the contact pressure is therefore low. This form of articu- lation,while imposing no constraints upon movement,di- minishes polyethylene wear to very low values. Measure- ment of retrieved bearings has shown a mean linear wear rate (combining both articular surfaces) of 0.03 mm/year, and even less (0.01 mm/year) if the knee had been func- tioning normally with no impingement [4, 5]. Further- more, the rate of wear is no more rapid in thin compo- nents (i.e., 3.5 mm) than in thicker ones. The use of thin polyethylene is advantageous, as bone stock is preserved.
Indications and Contraindications
The main indication for UKA is medial compartment os-
teoarthritis [6]. The anterior cruciate ligament should be
functionally intact [7]. We believe this requirement to be
paramount. It will be found that, if the ACL is intact, the
other requirements for success are also usually present
Chapter 51 · Mobile-Bearing Unicompartmental Knee Arthroplasty – D. G. Murray 323
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[8]. The fixed flexion deformity should be less than 15°.
The intra-articular varus deformity should be cor- rectable, indicating that the medial collateral ligament is not shortened.There should be full-thickness cartilage in the lateral compartment (best demonstrated by valgus stress X-rays taken with the knee in 20° of flexion) [9].At operation, a full-thickness ulcer is often seen in the carti- lage on the medial side of the lateral femoral condyle from impingement on the tibial spine; this is not a contraindi- cation.
We have shown that many of the contraindications proposed for fixed-bearing UKA are unnecessary for the mobile-bearing device [6]. In our practice, no knee is ex- cluded because of patellofemoral disease, except for the very rare case in which there is gross eburnation and grooving. In medial unicompartmental arthritis, exten- sive fibrillation and erosions are commonly found in the patellofemoral joint. We have found no correlation be- tween the state of the patellofemoral joint or the presence of anterior knee pain and the clinical outcome [10,11],and we have not had to revise a knee for patellofemoral pain.
Furthermore, we have shown by radiographic compari- son at an interval of 10 years that arthritis does not progress in the patellofemoral joint after UKA [11]. Nor is age a contraindication. The low morbidity of UKA is a clear advantage over TKA in elderly patients. In younger subjects, UKA can be recommended as no more likely to fail at 10-15 years than TKA and with the advantage that, should failure occur, revision to TKA is simple and has good results [13, 14]. We have shown that patients in their 50s have a 10-year survival rate better than 90% and that this is not significantly different from older subjects [13].
Moderate obesity and the presence of chondrocalcinosis have both been shown to be without adverse effect on long-term survival [15].
Avascular necrosis (AVN) is also an appropriate indi- cation [16] and,although the numbers are small,the long- term results of medial UKA for AVN are similar to those for osteoarthritis. Currently, we do not recommend the use of a mobile-bearing UKA in the lateral compartment because of the high dislocation rate [17]. However, we do believe that in the future there will be a role for a mobile- bearing UKA with a modified design and surgical tech- nique. Similarly, we do not recommend UKA after failed high tibial osteotomy (HTO), as the survival is about 66%
at 10 years [18]. Inflammatory arthritis is a definite con- traindication. Using our indications, about one in three knees needing replacement are suitable for UKA.
Instrumentation and Technique
Instrumentation and surgical technique are very impor- tant in unicompartmental replacement, the object of which is to restore the kinematics of the damaged com-
partment so that it functions in compliance with the re- tained articular surfaces and ligaments of the undam- aged compartment. Using a mobile-bearing implant en- sures that the prosthesis itself imposes no artificial con- straints, but the stability of such a device depends upon restoring ligament tension isometrically throughout the range of movement. In TKA, ligament balancing is achieved by ligament release; in UKA it is attained by placement of the artificial articular surfaces to match the anatomy of the ligaments which are never released.
In the first design of the Oxford Knee (phase 1),the fe- mur was prepared with a saw. Precise ligament balance was difficult to achieve,and the bearings occasionally dis- located. Since 1985, using the phase-2 and phase-3 instru- ments, the femur has been prepared with a guided power mill which can remove bone from the inferior surface of the femoral condyle in 1-mm increments, gradually in- creasing the gap between the articular surfaces in exten- sion until it is the same as the gap in flexion.When the gap is filled with a bearing of the right thickness, the liga- ments are restored to their normal tension and remain so throughout the range of movement. This results in nor- mal ligament function and a very low incidence of dislo- cation: The in vivo kinematics of the Oxford Knee, as- sessed fluoroscopically, were found to be identical to those of normal knees and substantially better than those after TKA [19].A meta-analysis of published results of the phase-2 implant, used for appropriate indications in the medial compartment, revealed a dislocation rate of 0.4%
(two of 551 UKA) [20].
Results
The designer’s own series of primary medial unicom-
partmental arthroplasty (phase 1 and phase 2) for an-
teromedial osteoarthritis consisted of 144 knees (patients
aged 35-90), one of which was lost to follow-up [6]. The
10-year survival was 98% [95% confidence limits (CI)
93%-100%]. The designer’s own results may need to be
treated with caution as susceptible to bias; however, there
are now a number of published independent series of 10
years or more with a similar survival rate. For example,
Svard and Price [21] reported a series of 420 medial UKA
with none lost to follow-up treated by three surgeons at a
non-teaching hospital in Sweden [22]. The 15-year sur-
vival was 94% (CI 86%-100%). One hundred twenty-two
of these had a follow-up of 10 years or more and were re-
viewed clinically; 92% had good or excellent results. In
Smith’s series of 135 medial UKA with none lost to follow-
up the 10-year survival was 94% (CI 84%-98%) [23]. In
Keyes’ series of 40 medial UKA with none lost to follow-
up the 10-year survival was 100% [24]. In these survival
studies revision for any cause was considered to be a fail-
ure. The results from these series are therefore as good as
the published results of TKA and better than the pub- lished results of fixed-bearing UKA [6].
By contrast, a survival of only 90% at 5 years was re- ported in 1995 for the phase-1 and phase-2 Oxford UKA enrolled in the Swedish Knee Arthroplasty Register (SKAR) [25]. There were 699 knees from 19 surgical cen- ters and they included medial and lateral replacements.
We were able to obtain data from 13 of these 19 centers and found 944 Oxford UKA, suggesting that the Register failed to recruit at least 25% of the knees. We found that the failure rate varied from center to center, from 0% to as high as 30%, and that centers implanting greater num- bers had lower failure rates. About 70% of the failures were in the first 2 years, and dislocation of a bearing was the most common cause.As the same implant was used in all cases, the poor early results must be attributed to wrong indications and/or inappropriate technique, occa- sioned, perhaps, by the effect of at least 19 “learning curves”. The 2001 report from the SKAR [26] confirms our observation that a surgeon needs to perform a rea- sonable number of Oxford UKA to be proficient.It also re- vealed that,in centers doing at least two UKAs per month, the 8-year survival of the Oxford UKA was 93%, margin- ally higher than that achieved by other UKA.
In 1998 the phase-3 implant was introduced, partly to address the problem of inconsistent results by making the operation simpler, and partly to facilitate a minimal- ly invasive approach. The operation is now performed through a short incision from the medial pole of the patel- la to the tibial tuberosity. There is minimal damage to the extensor mechanism,the patella is not dislocated,and the suprapatellar synovial pouch remains intact. As a result, patients recover much more rapidly. We have found that knee flexion, straight leg raising, and independent stair climbing are achieved three times faster after this proce- dure than after TKA and twice as fast as after open UKA [27]. With appropriate pain control the procedure can be done safely as a day case with substantial cost savings,and patients report less pain during the postoperative period than they suffered preoperatively [28, 29].
Conclusions
A study of postoperative radiographs has shown that the Oxford components can be implanted equally precisely through the limited approach and through the tradi- tional open incision [27], suggesting that the long-term results of the minimally invasive phase 3 should be as good as those of the phase 2. With the first 500 phase-3 Oxford UKAs implanted by two surgeons in Oxford the 5-year survival was 98% [30]. This demonstrates that at least at 5 years the survival of the phase 2 and phase 3 are similar. The functional results of the minimally invasive phase-3 UKA are better than those of the phase 2, how-
ever [31]. The greatest difference is in range of move- ment: With the phase 3 the average postoperative flexion (:130º) is about 10° higher than the preoperative flexion, whereas with phase 2 they are similar. We attribute the excellent functional results of both phase 2 and phase 3 to the accurate restoration of function in all the liga- ments, particularly the cruciate mechanism, and the additional improvement in phase 3 to the avoidance of damage to the extensor mechanism and the suprapatel- lar pouch.
We believe we have shown that, when the indications are met, the Oxford UKA is the treatment of choice for medial osteoarthritis of the knee. It provides the patient with the many advantages of UKA over TKA without in- creasing the risk of failure, at least in the first 15 years.
References
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324 VI . Implant Design
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