A 21 % conversion rate to total knee arthroplasty of a first-generation patellofemoral prosthesis at a mean follow-up of 9.7 years

We reported the mid- to long-term results of Richards’ II PFA placed in our centre between 1998 and 2007 in terms of survival, functional scores, number and type of complications and patient satisfaction. The survival with conversion to TKA as endpoint of 73 % (95 % CI, 57–93 %) at 10 years we found in our study is, taking the confidence interval in to consideration, comparable to published data on the Richards’ II PFA, which varies from 75 % (surgery for any reason, mean follow-up 10 [6–16] years) to approximately 84 % (conversion to TKA, mean follow-up 13 [2–31] years) [6, 7]. The ODEP benchmark of 90 % survival at 10 years, widely accepted as a minimal requirement for total hip and knee arthroplasty, is not met, although 90 % is still within the range of the confidence interval (57–93 %).

As mentioned in the introduction, the first generations of PFA experienced more complications than newer designs [8]. Our re-operation rate of 36 % (12 out of 33) is higher than the 25 % reported by Dy et al. [4]. In contrast, our patient satisfaction scores were good to excellent in 90 %, compared to 76–86 % in the literature [3, 9]. In the literature, mechanical complications were most commonly noted, followed by persistent pain and ongoing (tibiofemoral) osteoarthritis (TFOA) [4]. We saw the same type of complications but in a different order. Most common was TFOA (n = 5, 15 %), followed by pain and/or stiffness (n = 3, 9 %), mechanical complications (instability) (n = 3, 9 %) and (deep) infection (n = 1, 3 %). We tried to deduct if these re-interventions could have been prevented. For the TFOA cases (Table 1: cases 1R, 4R, 1L, 9L and 11L) there does not seem to be a causal relationship with the primary indication or previous surgery. In these cases further surgery occurred 5.6–7.2 years after implantation. The infection case (Table 1: case 11R) was probably a low-grade infection from the start, becoming apparent at 1.5 years after implantation with no apparent link with patients pre-operative history. However, this case was re-operated on multiple times because of tracking issues. The number of procedures are of course a risk factor for prosthetic joint infection (PJI). The pain and stiffness cases (Table 1: cases 6, 8 and 9R) all seem to have a more elaborate history that could influence the results of PFA, or any other type of surgery for that matter. But again there does not seem to be a direct causal link with patient history. Special consideration should be given to patellofemoral instability cases (Table 1: cases 2L, 3, 5 and also 11R). The only case (Table 1: case 3) in which instability was the indication was also unstable after implantation of the device. Apparently the constrained design was not enough to solve these pre-existing patellofemoral dislocation issues. Possibly the re-intervention (a re-alignment) could have been prevented when a re-alignment procedure was done at or before PFA implantation. Our assumption that it is important to adequately treat instability before PFA is highlighted by cases 20, 7L and 7R (Table 1), in which a history of medialising tuberosity transfer osteotomies led to a good outcome after PFA. In contrast, a Maquet procedure does not seem to have the same effect. This is understandable because in this procedure there is only advancement of the tuberosity with no influence on stability.

The remaining instability cases (Table 1: cases 2L, 5 and 11R) had no previous history of instability. Either this was not apparent at the time of surgery or there are surgeon-related and/or implant-related factors that contributed to the post-operative problems in these cases. This underlines the importance of proper surgical technique and possibly also reflects effects of low volume surgery. Cases 5 and 11R (Table 1) illustrate that once tracking problems exist it is not easy to resolve them by just revising a part of the complete prosthesis. As for possible implant-specific related issues, the high constrained design of the Richards’ II easily leads to assumptions that it is less sensitive to medio-lateral instability. But you can also argue that in the pursuit of proper tracking there is less freedom of play compared to the current more mobile, less constraint designs. Hypothetically in this design, moderate malalignment could lead to polyethylene (over)loading of the (lateral) facet and trochlea shoulder leading to subsequent wear and pain because of strain on the soft tissues and particle disease. Observing the little to no polyethylene wear in conversion cases up to 12 years after primary implantation, overloading and wear did not seem to be a big issue in the Richards’ II. Severe malalignment, however, could still lead to luxation, even in this design. As was shown in case 3 (Table 1). In the literature, more and more emphasis is being put on optimal implant positioning to achieve optimal tracking. Current designs use more sophisticated guiding instruments that mostly refer to the intramedullary femoral canal, anterior cortex, mechanical tibial axis, femoral epicondyles, direction of the native trochlea or a combination of these. Some authors remind us of the added value of navigation [10]. Most PFAs are based on TKA trochlea templates and anterior femoral cuts (“onlay”) or sinking the trochlea shield into the native trochlea (“inlay”). These different options and supporting philosophies allow for different ways of implantation and positioning control [11], but there is no consensus on what the optimal method or position should be. In theory the amount of constraint in the Richards’ II would make it less prone for implantation errors; but it seems that no matter which direction the patella is forced in, it will follow its own course. A number of cases (e.g. cases 5 and 11R) illustrate this point.

The V-shaped patellabutton is a concern for some authors because conversion to TKA means revising the patella as well [12]. To our knowledge, there is no discussion if the patella should be resurfaced or not in PFA, opposed to TKA [13, 14]. In any case, in our experience the patellabutton does not cause problems in revision cases. It can be easily removed at the level of the bone cement-polyethylene interface. The remaining pegs can also be removed without effort. In all cases we were left with a good bone surface to receive a new patellar implant.

The ideal candidate for PFA, as proposed by Walker et al. [15], should be 50–60 years old, in our cohort the average age was 47.7 years, so not all our patients fit this profile. Also, there where disproportionally more women (79 %) than men (21 %), over-representing women.

A common re-operation was conversion to TKA because of ongoing tibiofemoral osteoarthritis in the medial or lateral compartment [4]. Seven of the 33 (21 %) cases were converted to TKA at an average time of 5.5 years after implantation; five of which were because of TFOA. The number of conversions would probably be lower if we were able to predict which patients develop ongoing osteoarthritis. Presumably, in these patients signs of disease in other compartments were present when the indication for PFA was made, although not visible on X-rays. Pain in other areas of the knee than anterior is highly suggestive and should be investigated. Subclinical osteoarthritis can become apparent on magnetic resonance imaging (MRI), bone scintigraphy, single-photon emission computed tomography (SPECT) or arthroscopy [16‐18], which can help to establish a correct diagnosis. In any case, we highly discourage using PFA in patients who show any symptoms of disease in other compartments than patellofemoral.

Patient selection, proper surgical technique, implant design and positioning, as well as surgical volume, probably all played a role in achieving the high operation rate in this cohort.

Some potential limitations of the study have to be discussed. First, although comparable to other studies, the study sample is relatively small [4]. Second, we have a relatively large number of bilateral cases (18 out of 33; 55 %). Both the small sample and the large percentage of bilateral cases may interfere with the survival analyses [19, 12, 20, 21]. Finally we did not use a PF specific questionnaire as in other PF related studies [14]. Strength of this study is that it is a single surgeon series.

In conclusion we found a 10-year survival of the Richards’ II PFA of 73 % (95 % CI, 57–93 %), which is comparable to known data but does not seem to meet the ODEP criterion of 90 % survival at 10 years. Although re-operation rates are slightly higher than those published by other groups so are the satisfaction rates. We consider our results comparable to those reported in literature. According to current standards, implantation of this particular design should only be allowed in a controlled study environment. For this type of prosthesis, but we believe this can be extrapolated to current PFA designs, patient selection, proper surgical technique, implant design and positioning are crucial to optimise outcome. Seventy-one percent of the conversion-to-TKA cases were because of TFOA. We strongly suggest refraining from the use of PFA in knees that show any sign of disease in other compartments than patellofemoral.