Advancements in implant designs, particularly the trochlear component, has made Patellofemoral Arthroplasty or Kneecap Replacement Surgery a viable option for carefully selected patients suffering from patellofemoral arthritis. Epidemiologic studies generally indicate that isolated patellofemoral arthritis affects almost 10 percent of the population who are above 40 years of age. Another study reveals that females are twice as more likely than males to be having isolated anterior compartment degeneration & which is probably related to malalignment & subtle dysplasia.
More patients are likely to seek treatment as people age & increase the burden of arthritis. Additionally, younger patients between 30 – 50 years of age will continue to present cases of isolated patellofemoral arthritis. Conservative treatment operations like patellofemoral arthroplasty or PFA in short, will remain important alternatives to total knee arthroplasty in cases where non-operative interventions are ineffective. However, most patients suffering from patellofemoral arthritis can be symptomatically treated with non-operative modalities like the following.
Small percentage of patients would however still require surgical intervention when the treatments mentioned above fail to bring positive results. Surgical options for patellofemoral arthritis include the following:
Non-arthroplasty surgical procedures have historically provided inconsistent & mixed results with success rates ranging between 60 – 70 percent, especially among patients with advanced conditions of arthritis. Though TKA is able to provide reproducible results among patients with isolated patellofemoral arthritis, it is undesirable for those patients who are more interested in conservative & kinematic-preserving approach & which is particularly popular among young patients. It is because of these limitations that PFA is emerging as a much more mainstream option.
Like it is with any surgical procedure, prerequisite for a good outcome with PFA is proper selection of a patient. That is why results of PFA must therefore be interpreted in context of appropriate indications. Ideal candidates for PFA have isolated & non-inflammatory anterior compartment arthritis which usually results in pain & functional limitations which are persistent despite having put reasonable attempts at non-operative treatments. Good candidates for PFA must have only retro and/or peri-patellar pain which is invariably exacerbated by sitting with flexed knees, ascending or descending stairs & standing from a seated position. These symptoms should also be reproducible with squatting & also during patellar inhibition testing physical examination. Abnormal J-sign or Q-angle indicates dysplasia and/or significant maltracking, particularly among patients having previous history of patellar dislocations. Presence of these conditions would generally necessitate concomitant realignment surgery along with PFA.
With help of newer prosthesis designs, moderate maltracking can be effectively corrected with proper orientation of prosthesis & occasionally with a lateral release. Patellofemoral arthritis patients most often also have significant quadriceps weakness which can be effectively corrected through preoperative physical therapy in order to prevent functional limitations & prolonged postoperative pain. Radiographs should also prove to be consistent with isolated patellofemoral arthritis which is indicated by osteophytes on lateral & Merchant views & by narrowing of joint space. Narrowing within the lateral or medial compartments on weight-bearing sites would generally disqualify the patient from being a good candidate for PFA. Many rheumatologists would prefer to obtain a preoperative MRI – magnetic resonance imaging scan as well in order to further evaluate tibiofemoral compartments to search for evidence of reactive edema or chondral damage & which would eventually help guide proper treatment between TKA or PFA & bicompartmental arthroplasty. Previous images of arthroscopy are also especially valuable evidence in documenting extent of cartilage loss in anterior compartment & presence or absence of degeneration located anywhere else within the knee joint.
Patellofemoral arthroplasty procedure was first developed about 30 years back. Moreover, it remained somewhat controversial most of the time until recently due to high rates of failure which were seen with early inlay-style trochlear prosthetic designs. Contemporary onlay-style trochlear implants have however replaced the entire anterior trochlear surface & which are now more optimally positioned. As a result of this, patellofemoral arthroplasty has easily achieved high success rates along with good functional outcomes. Initial attempts at patellofemoral arthroplasty had utilized trochlear components which were inset into native trochlea in order to attempt to position the prosthesis flush within the surrounding trochlear articular cartilage. Characteristics of the resulting design proved problematic when it was coupled with the inclination of native trochlea & inherent anatomic variations among patients. This eventually made positioning of the component a challenging task in relation to articular surfaces which biased the component into internal rotation & predisposing it to higher rates of patellar subluxation, catching & maltracking.
Common characteristics of inlay-style patellofemoral arthroplasty prosthesis include the following:
Onlay-style trochlear prosthesis is designed to replace the entire anterior trochlear surface while alleviating many issues that were described above & which involved having to accept constraints of native aberrations that are common in the patient population. This type of design is generally applied to all patients regardless of their anatomic variations & is therefore much more versatile & suitable for common usage. However, characteristics of onlay-style prosthesis are listed below.
Though patient selection & proper surgical techniques are very important drivers of success in PFA, analysis of PFA results have shown disparity in early or mid-term failures which normally occurred as a result of patellar maltracking & instability, depending upon whether an inlay-style or onlay-style component was used. However, none of the studies directly compared inlay-style & onlay-style trochlear prostheses. Nevertheless, preponderance of evidence reveals lower revision rates & requirement of secondary surgery in order to address patellar maltracking & durability along with higher functional success rates with onlay-style prostheses. Although initially poorly understood, high revision & reoperation rates with inlay-style trochlear implant designs were generally attributed to poor patient selection, component malposition & soft tissue imbalance. However, many of the older & contemporary inlay-style PFA implants are no more in use today. In case patella tracking is satisfactory following PFA, primary mode of failure, if any, would be progressive tibiofemoral arthritis, irrespective of the form of trochlear prosthesis which was utilized.
Short-term complications which are associated with PFA are most frequently related to maltracking & catching. In contrast, long-term complications which require revision may occur in setting of a properly functional PFA. Revision rates have been found to be higher among obese patients & which is most likely due to a combination of factors including the following.
Studies which investigated use of role of revision PFA reported 14 failed first-generation inlay-style prostheses which were subsequently revised to second-generation onlay-style PFA implants. Primary failure modes which were found included the following.
However, no loosening was reported.
At a mean 5-year follow-up, ranging between 3 – 7 years, significant improvements were noticed in pain & function sub-scores. Some PFA patients displayed evidence of mild tibiofemoral arthritis at reoperation, which in a way predicted poor outcomes. Almost half of these patients were subsequently revised to TKA by the end of the final follow-up. However, no malpositioning, subluxation, wear or loosening was noted in any of these revision PFA prostheses. It was concluded that revision PFA using onlay-style can be a viable option when confronted with a failed inlay-style PFA, provided there is absence of degeneration located elsewhere within the knee joint. Additionally, though design characteristics of inlay-style prosthesis may have contributed to its clinical failure, in a way it also facilitated a relatively easy revision procedure due to bone-preserving nature of the earlier design.
However, little has been said about revision of PFA to TKA in contrast to conversion of unicondylar knee arthroplasty to TKA. There are only few reported results of failed PFAs revised to TKAs. Over 80 percent PFAs in a study failed in patients at a mean of 4 years post operation due to progression of arthritis alone or in combination with patellar catching & maltracking. However, significant improvements in clinical & functional assessment were noted without any evidence of maltracking, wear or failure of resultant reconstruction, were observed at a mean of 3 years. Outcomes of conversions to TKA were similar to the ones after primary TKA, but then only trochlear components were revised. However, these outcomes may not have been so optimal, in case revision of patellar components, were required as well.
Numerous studies have reported successful outcomes of TKA performed for isolated anterior compartment arthritis. Moreover, these cases showed good mid-term results in up to 90 percent of the patients. A retrospective study which compared outcomes of patients undergoing PFA or TKA at a mean 2.5-year follow-up found similar pain scores but PFA group faired significantly better on activity scores. However, quality comparisons of PFA to other treatments like TKA for isolated patellofemoral arthritis have still not been reported as of now. Just one randomized control trial is currently under progress evaluating PFA when compared to TKA within this scenario & is still expected to report results. Another recent meta-analysis compared complications with PFA & TKA operations for isolated patellofemoral arthritis. These researchers found an eight-fold higher possibility of revision & reoperation for all PFA cases when compared with TKA. However, when second-generation onlay-style prostheses were compared to TKA, no significant differences were found in pain, revision, reoperation, or in mechanical complications, indicating significant effects of implant design. Moreover, on sub-group analysis, first-generation inlay-style prostheses were found to have more than four-times higher rates of significant complications when compared with second-generation prostheses & which were likely biasing overall results. All these data generally indicate that the latest onlay-style PFA & TKA most likely have similar rates of complications within the assessed patient population.
Significant failure rates along with patellar tracking complications which earlier plagued inlay-style PFA designs have now been effectively minimized with the new generation of onlay-style PFA prostheses. PFA outcomes therefore can now be optimized with proper attention to the following factors.
Minimizing risk of patellar instability with onlay-style of PFAs has in fact enhanced mid-term & long-term results. This now only leaves progressive tibiofemoral arthritis as a primary failure mechanism for PFA patients & that too beyond 10 – 15 years of time.
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