A Resurgence for Patellofemoral Arthroplasty

    PFA is emerging as a more mainstream option for patients with patellofemoral osteoarthritis. This review focuses on the historical and contemporary results of PFA as influenced by advances in prosthetic design.


    Julie Shaner, MD, and Jess H. Lonner, MD


    Julie Shaner, MD, has no disclosures relevant to this article. Jess Lonner, MD has the following relationships to disclose: Zimmer, Blue Belt Technologies, CD Diagnostics, Healthpoint Capital, Wolters Kluwer Health, Lippincott Williams & Wilkins, Saunders/Mosby-Elsevier, American Journal of Orthopedics, Journal of Arthroplasty, The Knee Society.


    Epidemiologic studies indicate that isolated patellofemoral arthritis affects nearly 10% of the population over 40 years of age. [1] In one study, females were more than twice as likely as males to have isolated anterior compartment degeneration (24% vs. 11%), [2] probably related to subtle dysplasia and malalignment. [3]

    As the population ages and the burden of arthritis increases, [4] more patients will likely seek treatment for this condition. Additionally, as younger patients in their 30s through 50s continue to present with isolated patellofemoral arthritis, conservative operative treatments such as patellafemoral arthroplasty (PFA) will remain important alternatives to total knee arthroplasty (TKA) when non-operative interventions are ineffective.

    Most patients with patellofemoral arthritis can be treated symptomatically and with non-operative modalities, such as:

    • Anti-inflammatory medications
    • Physical therapy
    • Weight reduction
    • Bracing
    • Injections

    A small percentage of patients, however, may require surgical intervention if these treatments fail. Surgical options include:

    • Non-arthroplasty procedures, such as arthroscopic debridement, tibial tubercle unloading procedures, cartilage restoration, and patellectomy
    • Partial knee arthroplasty (PFA)
    • Total knee arthroplasty

    Historically, non-arthroplasty surgical treatments have provided mixed and inconsistent results, with success rates of 60-70%, especially in patients with advanced arthritis. [5] Although TKA provides reproducible results in patients with isolated patellofemoral arthritis, it may be undesirable for those interested in a more conservative, kinematic-preserving approach, particularly in younger patients.

    Due to these limitations, PFA is emerging as a more mainstream option. This review will focus on the historical and contemporary results of PFA as influenced by advances in prosthetic (specifically trochlear component) design.

    Indications for PFA

    As with any surgical procedure, a prerequisite for good outcomes with PFA is proper patient selection. Therefore, results of any series of PFA should be interpreted in the context of appropriate indications.

    The ideal candidate for PFA has isolated, non-inflammatory anterior compartment arthritis resulting in pain and functional limitations that are persistent despite reasonable attempts at non-operative treatments. Patients should have only retro- and/or peri-patellar pain that is exacerbated by stairs, sitting with the knee flexed, and standing from a seated position.

    Symptoms should be reproducible during physical examination with squatting and patellar inhibition testing. An abnormal Q-angle or J-sign indicate significant maltracking and/or dysplasia, particularly with a previous history of patellar dislocations. The presence of these findings may necessitate concomitant realignment surgery with PFA. With newer prosthesis designs, however, moderate maltracking can be corrected with proper orientation of the prosthesis and occasionally a lateral release.

    Patients with patellofemoral arthritis often have significant quadriceps weakness, which should be corrected with preoperative physical therapy to prevent prolonged postoperative pain and functional limitations.

    Radiographs should be consistent with isolated patellofemoral arthritis, indicated by joint space narrowing and osteophytes on the lateral and Merchant views. Narrowing within the medial or lateral compartments on weight-bearing views may disqualify that patient from PFA.

    We also prefer to obtain a preoperative magnetic resonance imaging (MRI) scan to further evaluate the tibiofemoral compartments for evidence of chondral damage or reactive edema, to guide treatment between PFA and bicompartmental arthroplasty or TKA. Previous arthroscopy photographs are especially valuable in documenting the extent of anterior compartment cartilage loss and the presence or absence of degeneration elsewhere in the knee.

    PFA Design Considerations

    Patellofemoral arthroplasty was first developed over 30 years ago, although it remained somewhat controversial until recently due to high failure rates seen with early (and even some contemporary) inlay-style trochlear prosthesis designs.

    Contemporary onlay-style trochlear implants replace the entire anterior trochlear surface and are more optimally positioned. As a result, high success rates and good functional outcomes are more easily achieved.

    Table 1 summarizes key design differences between inlay- and onlay-style trochlear components.

    Table 1. Generalized Design Characteristics of Inlay-Style and Onlay-Style Patellofemoral Prostheses





    Inset flush with native trochlea

    Replaces entire trochlea, perpendicular to AP axis


    Determined by native trochlea

    Set by surgeon, perpendicular to AP axis




    Proximal extension

    No further than native trochlear surface

    Extends further proximal than native trochlea

    Inlay-Style Prostheses

    Initial attempts at PFA utilized trochlear components inset into the native trochlea, attempting to position the prosthesis flush with the surrounding trochlear articular cartilage. The resulting design characteristics proved problematic when coupled with the inherent anatomic variations and inclination of the native trochlea.

    This made positioning of the component challenging relative to the articular surfaces and biased the component into internal rotation, predisposing to high rates of patellar maltracking, catching, and subluxation.

    Characteristics of inlay-style prostheses include the following:

    • The shapes of these components frequently do not match the shape of the trochlea, particularly in the situation of trochlear dysplasia. This can lead to malpositioning of the prosthesis, as it will not sit flush against all surfaces.
    • Several inlay prostheses have large radii of curvature. To avoid impingement of the implant on the anterior cruciate ligament or tibia by a proud inferior aspect of the prosthesis, flexion of these components may be necessary. This results in offset of the proximal aspect of the prosthesis from the anterior femoral cortex, causing catching and subluxation of the patella in the initial 15 to 30 degrees of flexion.
    • The rotation of the component is determined by the native trochlear orientation. A recent study by Kamath et al [6] examined trochlear inclination angles in 329 patients with either normal or dysplastic patellofemoral anatomy. Based on MRI scans, both groups had trochlear inclination angles averaging 11.4° and 9.4° of internal rotation, respectively, relative to the anatomic landmarks (anteroposterior and trans-epicondylar axes).
    • This finding explains the propensity for inlay-style trochlear components to malrotate internally, which predisposes to patellar maltracking and subluxation. Like internally rotated femoral components in total knee arthroplasty, internal rotation of the trochlear component in PFA effectively medializes the trochlear groove, increases the Q-angle, and puts tension on the lateral retinaculum, all which predispose to patellar maltracking and instability.
    • The narrow width and often deep constraining sulcus of some inlay-style trochlear components is more constraining to the patella with little accommodation for patellar tracking, which also increases the potential for patellar maltracking.
    • The proximal aspect of the inlay-style trochlear component does not extend proximal to the trochlear articular margin. This often results in the patella not being engaged in the trochlear component when the knee is in full extension, particularly in patients with patella alta. As the knee flexes, the patella transitions onto the trochlear component, which may cause catching and subluxation, particularly if the trochlear component is flexed, offset proximally, and internally rotated.

    Onlay-Style Prostheses

    Onlay-style trochlear prostheses replace the entire anterior trochlear surface, alleviating many of the issues described above when having to accept the constraints of native anatomic aberrations common in this patient population. This design can be applied to all patients, regardless of anatomic variations, and is therefore more versatile and suitable for general use.

    Characteristics of onlay-style prostheses include the following:

    • Most onlay prostheses have anatomic radii of curvature, ensuring the prosthesis sits flush with the anterior femoral cortex proximally and the articular cartilage above the intercondylar notch distally.
    • The rotation of the trochlear component is determined by the surgeon intraoperatively based on anatomic landmarks, similar to total knee arthroplasty. The component is positioned perpendicular to the anteroposterior axis (Whiteside’s line) and parallel to the transepicondylar axis. This facilitates patellar tracking and eliminates the effect of native trochlear rotation seen with inlay prostheses.
    • Onlay prostheses are wider, and less constraining than inlay designs, thus allowing greater excursion of the patella throughout the arc of motion.
    • Onlay prostheses often extend further proximal than the native trochlear cartilage and are positioned flush against the anterior femoral cortex. This eliminates the catching common to inlay designs, and also keeps the patella engaged in the trochlea even in full extension.

    Results of Patellofemoral Arthroplasty

    Primary Patellofemoral Arthroplasty

    Although patient selection and sound surgical technique are important drivers of success in PFA, an analysis of results of PFA have shown a disparity in the early- and mid-term failures that occur as a result of patellar instability and maltracking, depending on whether an inlay- or onlay-style component is used.

    Tables 2 and 3 contain the cumulative results of published series of inlay- and onlay-style trochlear prosthesis designs, respectively.

    Table 2. Published Results of Inlay-Style Patellofemoral Arthroplasty Prostheses

    Series (Year) Implant No. of PFA’s Age in years (range) Duration of follow-up in years (range) % of good/ excellent results % Revised

    Blazina (1979)

    Richards Types I & II


    39 (19-81)

    2 (0.6-3.5)



    Krajca (1996)

    Richards Types I & II


    64 (42-84)

    5.8 (2-18)



    Arciero (1988)

    Richards Type II (14); CFS-Wright (11)


    62 (33-86)

    5.3 (3-9)



    De Winter (2001)

    Richards Type II


    59 (22-90)

    11 (1-20)



    Kooijman (2003)

    Richards Type II


    50 (20-77)

    17 (15 – 21)



    van Jonbergen (2010)

    Richards Type II


    52 (NA)

    13.3 (2-30.6)



    Cartier (1990)

    Richards Types II & III


    65 (23-89)

    4 (2-12)



    Cartier (2005)

    Richards Types II & III


    60 (36-81)

    10 (6-16)



    Argenson (1995)



    57 (19-82)

    5.5 (2-10)



    Argenson (2005)



    57 (21-82)

    16 yrs (12-20)



    Van Wagenberg (2009)



    63 (31-81)

    4.8 (2-11)



    Tauro (2001)



    66 (50-87)

    7.5 (5-10)



    Smith (2002)



    72 (42-86)

    4 (0.5-7.5)



    Lonner (2004)



    38 (34-51)

    4 (2-6)



    Merchant (2004)



    49 (30-81)

    3.8 (2.3-5.5)



    Charalambous (2011)



    64 (47-84)

    2.1 (0.4-5)



    Sisto (2006)



    45 (23-51)

    6 (2.6-10)



    Table 3. Published Results of Onlay-Style Patellofemoral Arthroplasty Prostheses

    Series (Year) Implant No. of PFA’s Age in years (range) Duration of follow-up in years (range) % of good/ excellent results % Revised

    Lonner (2004)



    44 (28-59)

    0.5 (0.1-1)



    Ackroyd (2007)



    68 (46-86)

    5.2 (5-8)



    Starks (2009) Avon 37 66(30-82) 2 (NA) 86 0
    Leadbetter (2009) Avon 79 58 (34-77) 3 (2-6) 84 6.3
    Gao (2010) Avon 11 54 (46-74) 2 (0.5-4) 100 0
    Odumenya (2010) Avon 50 66 (42-88) 5.3 (2.1-10.2) NA 4
    Mont (2012) Avon 43 29 (27-67) 7 (4-8) NA 12
    Beitzel (2012) Journey PFJ 22 46 (26-67) 2 (NA) NA 4.5
    Lonner (unpub) Gender Solutions 168 46 (28-82) 3 (3 mos-6 yrs) NA 1


    No studies have directly compared inlay- and onlay-style trochlear prostheses. However, the preponderance of the evidence shows lower revision rates and need for secondary surgery to address patellar maltracking, as well as higher functional success rates and durability, with onlay-style prostheses. [9-24]

    While initially poorly understood, high reoperation and revision rates with inlay-style trochlear designs were often attributed to poor patient selection, soft tissue imbalance, and component malposition. In those series, the components were likely positioned flush with some, but not all, articular surfaces (due to morphologic mismatches between surface anatomy and trochlear implant) and internally rotated due to the native trochlear inclination. [9-24]

    Again, while poorly defined in the published series, the disproportionately low rates of satisfactory outcomes can likely be attributed at some level to trochlear component design features. [9-24] This helps explain data published in the Australian National Joint Registry showing that the 5-year cumulative revision rate was more than 20% for inlay-style prostheses and less than 10% for onlay-style designs. [8]

    Series reviewing the results of inlay-style implants have reported an incidence of patellar maltracking ranging between 17% and 36%. [5,9,12,20,29] Other studies that have reviewed the experience with different onlay-style trochlear designs in PFA have found a considerably lower incidence of patellar maltracking, typically less than 1%. [5,25-27,30,40]

    Several of the older and contemporary inlay-style PFA systems are no longer in use today. If patella tracking is satisfactory after PFA, the primary mode of failure will be progressive tibiofemoral arthritis, irrespective of the type of trochlear prosthesis utilized.

    Late Complications of Primary Patellofemoral Arthroplasty

    Short-term complications of PFA are related most frequently to patellar catching and maltracking.

    In contrast, late complications requiring revision may occur in the setting of a well-functioning PFA. Revision rates have been shown to be higher in obese patients, [17] likely due to a combination of the following factors:

    • Progression of tibiofemoral arthritis is the most common reason for long-term “failure” after successful PFA. In one series, 25% of patients at 15 years required additional surgery for progressive arthritis. [13] Two other series also found radiographic evidence of progressive degeneration in over 20% of knees. [25-29] Similarly, Nicol et al [32] found a 12% revision rate for symptomatic tibiofemoral arthritis at a mean of 55 months.
    • These authors also observed that the indication for primary PFA was osteoarthritis in all of the revised patients, whereas none of the patients with trochlear dysplasia as the primary etiology were revised. Other authors have also found that patients with primary trochlear dysplasia tend to have better long-term outcomes than those with primary osteoarthritis. [19-27]
    • Loosening is an infrequent cause of late revision in most series. [7,25] Kooijman found a loosening rate of 2% of prostheses at a mean of 15 years. [13]
    • Despite relatively good mid-term clinical results in one study, [18] Argenson found in a subsequent follow-up study of the same patients that the extended survivorship declined significantly, with 58% survivorship at mean 16 years postop. [19]
    • The majority of these patients were revised for progression of arthritis (25%) or loosening (14%). Most trochlear components that were revised for aseptic loosening were cementless designs. Cemented components fared substantially better.

    Revision Patellofemoral Arthroplasty

    One study has investigated the use of role of revision PFA. Hendrix et al [7] reported 14 failed first-generation inlay-style prostheses that were revised to a second-generation onlay-style PFA implant. The primary failure modes were:

    • Component malposition
    • Patellar subluxation/catching
    • Polyethylene wear
    • Overstuffing

    No loosening was reported. [7]

    At a mean 5-year follow-up (range 3 to 7 years), significant improvements were noted in the Bristol Knee Scores, as well as its pain and function sub-scores. Five patients had evidence of mild tibiofemoral arthritis at reoperation, which predicted poorer outcome. Two of these five patients were revised to TKA by final follow-up. No malposition, loosening, wear, or subluxation was noted in any of the revision PFA prostheses. [7]

    Hendrix et al concluded that revision PFA using an onlay-style PFA is a viable option when faced with a failed inlay-style PFA, provided there is no evidence of degeneration elsewhere within the joint. [7] In addition, although the design characteristics of the inlay prosthesis likely contributed to its clinical failure, it also facilitated a relatively easy revision due to the bone-preserving nature of the early design.

    In contrast to conversion of unicondylar knee arthroplasty to TKA, little has been written about revision of PFA to TKA.

    Lonner et al [33] reported the results of a series of failed PFAs revised to TKA. Twelve PFA in 10 patients failed at a mean of 4 years postoperatively due to progression of arthritis alone or in combination with patellar maltracking and catching. Significant improvements in the clinical and functional Knee Society scores after revision were noted, with no evidence of wear, maltracking, or failure of the resultant reconstruction at a mean of 3 years.

    In this series, outcomes of conversion to TKA were similar to those after primary TKA; however, only the trochlear components were revised. [33] Outcomes may not have been as optimal if the patellar components required revision as well.

    Patellofemoral Arthroplasty vs. Total Knee Arthrolasty for Isolated Patellofemoral Arthritis

    Several studies have reported successful results of TKA for isolated anterior compartment arthritis, with good mid-term results in up to 90% of patients. [34,36] One retrospective study compared outcomes in 45 patients undergoing PFA or TKA at a mean 2.5-year follow-up. [37] The authors found similar Knee Society and pain scores, but the PFA group had significantly higher activity scores.

    However, high-quality comparisons of PFA to other treatments, including TKA, for isolated patellofemoral arthritis have not been reported to date. One ongoing randomized controlled trial is currently evaluating PFA compared with TKAin this scenario, and is expected to report results soon. [38]

    A recent meta-analysis of 28 studies compared complications with PFA and TKA performed for isolated patellofemoral arthritis. [39] The authors found an eight-fold higher likelihood of reoperation and revision for all PFA compared with TKA.

    However, when comparing second-generation onlay-style prostheses only, no significant differences in reoperation, revision, pain or mechanical complications were found, indicating a significant effect of implant design. [39] On subgroup analysis, first-generation inlay-style prostheses had more than four times higher rates of significant complications than second-generation prostheses, likely biasing the overall results. [39]

    These data indicate that modern onlay-style PFA and TKA likely have similar rates of complications in this patient population.


    The significant failure rates and patellar tracking complications that plagued early inlay-style PFA designs have now been minimized with the modern generation of onlay-style prostheses. PFA outcomes can be optimized with:

    • Proper patient selection
    • Meticulous surgical technique
    • Selection of an onlay-style implant that can be positioned perpendicular to the AP axis of the femur

    Minimizing the risk of patellar instability with onlay-style PFAs has enhanced mid-and long-term results, and leaves progressive tibiofemoral arthritis as the primary failure mechanism beyond 10 to 15 years.

    Author Information

    Julie Shaner, MD, and Jess H. Lonner, MD, are from The Rothman Institute, Philadelphia, Pennsylvania.


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