Summary
Isolated patellofemoral osteoarthritis is present in ap- proximately 10% of patients presenting with symptomatic degenerative knee joint disease.The current gold standard for treatment of end-stage disease after failure of conser- vative measures is total knee arthroplasty.The early results of isolated patellofemoral arthroplasty have been disap- pointing, but newer designs and an appreciation of the need to balance the soft tissues hold out the hope that a less invasive procedure may achieve results that match those of total knee arthroplasty. Concerns remain about progres- sion of disease to the medial tibiofemoral compartment.
Introduction
Approximately half of patients with degenerative arthri- tis of the knee have involvement of the patellofemoral joint. Given the female predilection for patellofemoral maltracking and the known association with premature patellofemoral osteoarthritis,it is perhaps surprising that Davies et al. noted in a population of symptomatic os- teoarthritic knees presenting over the age of 60 that 18.5%
of men and 17.1% of women had isolated patellofemoral disease as compared with 4.5% of men and 10% of women between the ages of 40 and 60 [1]. There is no doubt that isolated patellofemoral disease is a more common phe- nomenon than many clinicians realise and that it occurs more frequently than isolated lateral tibiofemoral disease.
This review looks at the various arthroplasty options for isolated patellofemoral osteoarthritis.
Anatomy
The patellofemoral joint includes the trochlear groove and the entire extensor mechanism of the knee, namely the quadriceps tendon,patella,and patellar ligament.The patella is a sesamoid bone that acts as a marker for the alignment of the extensor mechanism. The trochlear groove and an arch of articular cartilage around the in- tercondylar notch make up the femoral side of the joint.
Except in deep flexion, the tibial articular surface comes into contact with a different part of the femur than the patella does, and the majority of intercondylar notch os- teophytes result from patellofemoral disease.In many pa- tients with patellofemoral osteoarthritis the problem is secondary to trochlear dysplasia (⊡ Fig. 53-1). This may vary from a slightly shallow groove to an actual dome.
This will inevitably distort the anatomy and kinematics to a significant degree [2].
Kinematics
The movements of the patellofemoral joint are complex and have been reported by Goodfellow et al.[3].In full ex- tension only the distal part of the patella articular surface is in contact with the femoral groove, and as flexion pro- ceeds the contact area on the patella sweeps proximally until 90° of flexion, when the proximal part is in contact with the distal groove. From 90° of flexion the odd facet articulates with the lateral edge of the medial femoral condyle, and the lateral facet articulates with the medial edge of the lateral femoral condyle. The medial facet lies in contact with the synovium overlying the anterior cru-
⊡ Fig. 53-1. Severe trochlear dysplasia
ciate ligament. In deep flexion the patella effectively bridges the intercondylar notch, and at 135° of flexion the patella articulates with parts of both the medial and lat- eral femoral articular surfaces that also come into contact with the anterior meniscal horns (⊡ Fig. 53-2).
Indications for
Patellofemoral Arthroplasty
Patellofemoral arthroplasty should be considered for patients with isolated patellofemoral arthritis who have anterior knee pain uncontrolled by conservative and medical measures. It is very important to exclude tibiofemoral disease, inflammatory disorders, and re- ferred pain (especially from the hip).There is no firm evidence base in the literature to support or refute inclu- sion or exclusion on the grounds of age or weight.It is im- portant for patients to understand that isolated patello- femoral replacement is experimental, and not the “gold standard” operation, i.e., total knee arthroplasty. In our practice we have treated patients as young as 45 years of age with patellofemoral replacement as an alternative to patellectomy with success. In this young age-group it is essential that the patients show a positive attitude to treat- ment.The results are predictably bad in patients who have a significant psychological component to their pain. We emphasize that the operation may require revision to a to- tal knee arthroplasty in the future. Usually, the patients are in the same age range for total knee replacement, but the isolated replacement preserves the anterior cruciate ligament and allows for full flexion. Its success postoper- atively depends on building up the quadriceps muscle.
Arthroplasty
The patellofemoral joint has been the ‘Cinderella’ of knee arthroplasty. It is obvious from a glance at some of the early total knee implants that no real consideration was given to the patellofemoral joint. The Freeman-Samuel- son implant had no trochlear groove. The Attenborough
had a very short anterior femoral flange, and the sphero- centric implant completely disregarded the patello- femoral joint. Arthroplasty options for isolated patello- femoral arthritis are:
1. Total knee arthroplasty with patella resurfacing 2. Total knee arthroplasty without patella resurfacing 3. Isolated hemiarthroplasty patella resurfacing 4. Patellofemoral replacement
Total knee arthroplasty with patella resurfacing has,in ef- fect, created the gold standard [4, 5], with good or excel- lent results that match those of total knee arthroplasty for tibiofemoral disease. There are, however, persisting con- cerns about the scale of surgery to deal with what is, in ef- fect, an osteoarthritic process confined to the anterior compartment. One novel approach has been to reduce the morbidity of surgery by leaving the patella unresur- faced in total knee arthroplasty for isolated patello- femoral disease, as an extrapolation of the as yet unre- solved and long-running debate about patella resurfacing in total knee arthroplasty for tibiofemoral disease.Bever- land has shown good results using the LCS mobile-bear- ing total knee implant with simple patella débridement, but leaving the patella unresurfaced [6].
Whilst the concept of unicompartmental replacement has recently gained considerable popularity following publication of excellent 10-year results for the treatment of isolated medial tibiofemoral disease, there remains a considerable degree of scepticism about the advantages of isolated patellofemoral joint arthroplasty, for a variety of reasons:
1. The already established excellent long-term results both in terms of quality of pain relief and long-term survival for conventional total knee replacement 2. Concern about progression of the osteoarthritic
process to the tibiofemoral joint - particularly the me- dial compartment
3. Concerns about persisting extensor mechanism insta- bility, given that a large majority of patients who will be eligible for isolated patellofemoral replacement pre- sent with end-stage extensor mechanism malalign- ment and maltracking
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90°
45°
20°
Odd facet 135 degrees
⊡ Fig. 53-2. Contact area of patellar surface at varying angles of knee flex- ion. (After [3])
the age of 40.
Biomechanics
The patellofemoral joint has to withstand very consider- able contact forces with increasing knee flexion. Huberti and Hayes [8] noted that in extension the patella was sub- jected to 1.5 times body weight, and this increased to 7 times body weight in deep flexion. The contact pressure was further increased by a larger Q-angle, which tended to cause skewing of the patella. There is a part of both the medial and lateral femoral condyle which comes into con- tact both with the anterior horn of the relevant meniscus in extension of the knee and with the odd and lateral facets of the patella in deep flexion.As it is not possible to resurface the whole of the trochlear area which comes into contact with the patella in deep flexion, it is therefore imperative that any implant at least match the normal femoral geometry at the distal end of the femoral com- ponent,to allow a smooth transition from bearing surface to articular surface, as the patella will effectively transfer on to the articular cartilage of the femoral condyles. The component will also require smooth radiused edges that can be fitted flush with the femur to prevent meniscal
‘catching’ and inevitably pain. The transition from im- plant to articular surface is aided by soft-tissue infill - particularly on the patellar component, as described by Cameron [9].
Particularly with an all-polyethylene component, the significant loading on the patella in flexion may result in bending in deep flexion when bridging the intercondylar notch,thus increasing the potential for loosening and dis- location.
In addition to high contact stresses, the relative lack of congruency of the normal patellofemoral joint results in relatively low stability, and it is essential that any trochlear component is properly aligned in the vertical orientation and that appropriate soft-tissue balancing is performed to ensure satisfactory tracking of the patella [10-12].
One way to reduce potential patellar instability would be to increase the depth of the trochlea, with consequent reduction in the facet angle of the patella. This produces
Patellar Hemiarthroplasty
The first recognized patella resurfacing device was intro- duced by McKeever in 1949 [14]. This consisted of a met- al anatomical shell with slightly concave medial and lat- eral articular facets,which were asymmetric.Fixation was achieved with a single screw.The prosthesis was not sided but was merely turned upside down depending on whether the right or the left knee was being operated on.
McKeever reported his early results in 1955, but inter- pretation of data was complicated by the wide indications for surgery; e.g., six patients had rheumatoid arthritis, and additional surgical procedures including menisecto- my and synovectomy were included.
In 1992, Harrington [15] reported use of the implant
“as a salvage procedure for severe chondromalacia,” indi- cating reasonable long-term results with a mean follow- up of 8.1 years. There was no evidence of patellofemoral instability or prosthetic loosening.
A new patella prosthesis based on anatomical dimen- sional studies was reported by Aglietti [16]. For the first time his group considered cement fixation and also the option of a plastic component for use with conventional total condylar knee replacement. Their results using a metallic patella hemiarthroplasty were disappointing at medium term follow-up,however,probably on account of the dome shape, which was not congruent with the un- resurfaced trochlea and resulted in high contact pressure.
Patellar hemiarthroplasty has now fallen into disrepute.
If the ongoing debate about resurfacing of the patella at the time of total knee arthroplasty continues unre- solved,then it is interesting to speculate as to whether lat- er advocates of patellofemoral arthroplasty may suggest that the patella component be left unresurfaced after ap- propriate débridement.
Patellofemoral Arthroplasty
Concern about the use of patellar hemiarthroplasty in patients with trochlea changes led to the development of patellofemoral arthroplasty. The ideal features of a patellofemoral replacement are:
2. The trochlear component should not encroach on the intercondylar notch,which could result in injury to the anterior cruciate ligament.
3. There should be a smooth anatomical transition from the distal part of the trochlear component onto the ar- ticular cartilage of the two femoral condyles,to permit movement of the patellar component from the distal part of the trochlear prosthesis onto the articular car- tilage of the femoral condyles in deep flexion.
4. It should be possible to achieve minimal femoral bone resection to allow the implant to sit flush on the ante- rior femoral cortex.
5. It should be possible to determine correct rotation of the femoral trochlear component, as well as vertical alignment.
The Bechtol patella I system was introduced by the then Richards medical company in 1974,and the modified type II system was introduced in 1976 [17].This was a very con- strained prosthesis with a deep metal trochlear groove that tapered to a point at the apex of the intercondylar notch, and a matching patella implant (⊡ Figs. 53-3, 53-4).
In 1975 Lubinus introduced his own patellofemoral endo- prosthesis, which was more anatomical and attempted to reproduce the shape of the anterior aspect of the distal fe- mur [18].Unlike the Richards patella mod I,II,and III sys- tems, this involved the use of a sided trochlear implant.
Several patellofemoral prostheses have been developed in France, perhaps the best-known being the autocentric prosthesis [14].
The initial reviews were not encouraging. Study num- bers were small with short follow-up and the results did not come close to matching those of total knee arthroplasty [12, 17,19-23].Of the larger series,Cartier reported on 72 arthro- plasties, reviewed at between 2 and 12 years following im- plantation with a relatively short average follow-up of 4 years [11]. The use of Smith and Nephew mod III prosthe- ses demonstrated good or excellent results in 85% of cases.
Interpretation of the results was rendered difficult,howev- er, because in 36 of the cases a concomitant unicompart- mental tibiofemoral replacement was also performed.
Cartier emphasized that extensor mechanism realignment should be carried out at the time of arthroplasty surgery.It is of note that he performed 27 lateral retinacular releases and 34 tibial tubercle transpositions, utilizing a very con- strained prosthesis, but persisting lateral subluxation was noted in only two patients following surgery. This was in contrast to the original review by Blazina et al. of 57 im- plants, 20 of which had to undergo revision procedures to correct lateral patellar maltracking [17].
In 2001 the Bristol group in the UK presented a prospective review of the outcome of 76 Lubinus arthro- plasties with a mean follow-up of 7.5 years [24]. The clini-
cal outcome was satisfactory in only 45% of cases using the Bristol knee scoring system.Interpretation of the results is confused by virtue of the fact that in the majority of cases the sided femoral trochlear component was reversed, so that a left-sided component was used on the right side and vice versa. This was stated by the authors to have opti- mized patellofemoral tracking. Again, meticulous atten- tion was paid to patellar tracking at the time of surgery, with a secondary procedure (usually a lateral retinacular release) being performed in 4% of cases.At the time of re- view patellar malalignment was noted in 32% of patients and was the most common complication. Some concern was also expressed about the possibility of progression of medial tibiofemoral disease. Following publication of these results they abandoned the procedure.
Argenson reported a medium-term follow-up of 183 patellofemoral arthroplasties in 1994 [10].However,as 104 of these patients also underwent concomitant unicom- partmental tibiofemoral arthroplasty, only 79 implants were available for study at an average follow-up of 5.5
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⊡ Fig. 53-3. Femoral component Smith & Nephew Mod III arthroplasty in situ
⊡ Fig. 53-4. Postoperative skyline view of Smith & Nephew Mod III arthroplasty
years. They identified a bias in favor of patients with os- teoarthritis secondary to extensor mechanism dysplasia and fracture, and recorded a much higher failure rate in patients treated with primary patellofemoral os- teoarthritis.As a consequence they recommended patello- femoral arthroplasty for patients with secondary patello- femoral osteoarthritis.
As a result of dissatisfaction with the Lubinus implant, Ackroyd and the Bristol group developed the Avon im- plant, which was based on the anterior compartment of the Kinemax total knee implant [25] (⊡ Fig. 53-5). Instru- mentation was devised to enable the surgeon to implant the prosthesis with a greater degree of accuracy, in terms of both anterior positioning, to avoid “overstuffing” the anterior compartment, and femoral rotation. The first was implanted in September 1996, and they recently re- ported on their experience with 360 implants,59 of which have been in place for 5 years [26]. The incidence of patel- lar maltracking was only 4% and disease progression to the tibiofemoral joint 7%. The functional result matched those of current total knee arthroplasty designs.The most common complication was progression to symptomatic medial tibiofemoral disease, noted in 6% of cases. They also reported on a subgroup of 63 arthroplasties in younger patients under the age of 55, admittedly with a very short follow-up (mean 24 months). The early results matched those seen in older patients. This is the authors’
current preferred implant in cases of isolated patello- femoral osteoarthritis.
The dramatic increase in the use of unicompartmen- tal tibiofemoral implants, fuelled partly by long-term re- views suggesting results that match the best of total knee arthroplasty results with a much more conservative pro- cedure,have resulted in a rekindled interest in the concept of isolated patellofemoral arthroplasty. Long-term non- inventor reviews are awaited with interest, but it would appear that with the newer designs it is possible to achieve short- to medium-term results that match those of estab- lished total knee arthroplasty designs, with a less morbid
surgical procedure and a more rapid recovery. To put it into context, it is important to record that the total UK sales of the Avon implant in 2003 amounted to only 603 implants. It would appear that earlier concerns about the high level of residual patellar instability following use of these implants have been resolved, largely as a result of careful attention to extensor mechanism balancing at the time of the primary procedure. As with all unicompart- mental replacements,there are persisting concerns about progression of disease to other compartments in the knee, and there does appear to be an increased liability to progression of medial tibiofemoral disease.Given the Ox- ford unicompartmental knee replacement group’s data, which suggest a link between medial patellofemoral facet osteoarthritic change and medial tibiofemoral unicom- partmental disease, caution is perhaps appropriate when one is confronted with a patient who has isolated medial patellofemoral osteoarthritis (⊡ Fig. 53-6).
There are no hard data with regard to ease of revision and conversion to total knee replacement, but there is ev- idence [22] to suggest that revision of the femoral com- ponent presents little difficulty and that revision of the patellar component presents no additional difficulties over and above those of total knee arthroplasty. The re- cent development of minimally invasive surgical tech- niques may ultimately offer reduced morbidity when a patellofemoral prosthesis is implanted,but the need to ac- cess both the trochlear and the retropatellar surfaces without the ability to “decompress” the extensor mecha- nism by resection of distal, femoral, and proximal tibial bone leaves little scope for significant developments in minimally invasive access.
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⊡ Fig. 53-5. Intraoperative view of Avon patellofemoral joint replace- ment
⊡ Fig. 53-6. Medial patellofemoral osteoarthritis
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