At present, patellofemoral arthroplasty usually is done only after failure of a lengthy period of nonoperative treatment or failure of more conservative surgical procedures. Pain localized to the patellofemoral articulation that occurs during daily activities and is unresponsive to nonsteroidal anti-inflammatory drugs, injection therapy, and activity modification is a good indication for patellofemoral arthroplasty. The ideal candidate for patellofemoral arthroplasty is typically a patient in his or her fifties who has debilitating, isolated patellofemoral arthritis refractory to conservative treatment and no patellar malalignment. Older patients with isolated patellofemoral arthritis may fare better with a total knee arthroplasty, but we know of no published data showing age-dependent outcomes of patellofemoral arthroplasty.
Improved outcomes have been reported with patellofemoral arthroplasty for posttraumatic arthritis, primary patellofemoral osteoarthritis, and patellofemoral dysplasia without malalignment. Patients with posttraumatic arthritis for whom patellectomy is considered should be evaluated for patellofemoral arthroplasty. Primary patellofemoral arthritis includes Outerbridge type-IV chondromalacia of the patella and/or trochlea. It is important to note that tibiofemoral arthritis progresses more commonly in patients with primary osteoarthritis than with posttraumatic arthritis or dysplasia14. Malalignment is most often determined with use of the quadriceps angle (Q angle). The Q angle is the angle formed by the intersection of a line drawn from the anterior superior iliac spine to the center of the patella and the projection of a line drawn from the tibial tubercle to the center of the patella. Angles of >15° in men and 20° in women are considered abnormal. Any condition that increases the Q angle increases the lateral displacement forces on the patella and possibly leads to subluxation or dislocation. Patellofemoral arthroplasty alone cannot be expected to correct patellar malalignment, which is not an indication for the procedure. Mild patellar tilt or subluxation can be corrected at the time of patellofemoral arthroplasty with lateral retinacular release, medialization of the patellar component, and partial lateral facetectomy. If malalignment exists, it should be corrected before patellofemoral arthroplasty. No particular patellar or trochlear wear pattern has been identified as a contraindication to patellofemoral arthroplasty, but the prosthesis should address lesions in their entirety without extension into the femoral condyles.
The progression of tibiofemoral arthritis is the most common cause of revision to total knee arthroplasty, emphasizing the fact that tibiofemoral arthritis is a principal contraindication to patellofemoral arthroplasty. Inflammatory arthropathies by nature involve the entire joint, and patellofemoral arthroplasty currently is contraindicated in patients with these conditions, including patients with chondrocalcinosis, because of progressive tibiofemoral arthritis and painful synovitis. Because one purposed benefit of patellofemoral arthroplasty is retention of normal tibiofemoral kinematics, intact ligaments and menisci without tibiofemoral instability are reasonable prerequisites to patellofemoral arthroplasty; however, there is no consensus that cruciate deficiency or previous meniscectomy causes poor outcomes. Patellofemoral arthroplasty is not indicated in severe coronal plane deformity of the knee (valgus of >8° or varus of >5°) unless the deformity is corrected with an osteotomy before patellofemoral arthroplasty15. In the sagittal plane, 120° of free flexion with <10° of flexion contracture is recommended. Knee stiffness must be critically assessed because these patients have a high rate of previous surgical procedures, which increases the prevalence of arthrofibrosis or patellar height aberrations. Patients with patella baja from quadriceps muscle atrophy or patellar tendon scarring are not good candidates for patellofemoral arthroplasty.
Experimental models have shown that patellofemoral joint reaction forces are up to 3.3 times body weight at 60° of knee flexion and 7.8 times body weight at 130°16. Although there are few data correlating patellofemoral arthroplasty with body mass index (BMI), a general consensus is that patellofemoral arthroplasty should be avoided in obese patients to prevent overloading the implant. Recently, van Jonbergen et al.17 showed a higher rate of revision to total knee arthroplasty in obese patients (a BMI of >30) than in those with a lower BMI. Primary diagnosis, age, and sex did not significantly affect revision rate in their series17.
Patient History
Patients with patellofemoral arthritis typically describe anterior or retropatellar knee pain that is exacerbated with activities that preferentially load the patellofemoral articulation. These provocative activities usually involve ascending or descending stairs, walking on uneven surfaces, and kneeling or squatting. Patients often describe preferring to sit with the legs extended rather than flexed. Frequently, patients do not recognize the association of their pain with knee posture, but they report pain with long car rides or prolonged sitting and activities associated with lengthy periods of knee flexion. Often there is a history of crepitus and effusions, but it also is important to inquire about subluxation or dislocation events that could indicate patellar malalignment.
Physical Examination
It is critical to evaluate both lower extremities from the pelvis to the feet. Limb length, alignment, quadriceps muscle atrophy, and foot alignment should be evaluated. Particularly, assessing the Q angle and patellar tracking quickly determines whether patellofemoral arthroplasty should be included in the treatment algorithm. Patients with large Q angles (>15° in men and >20° in women) or signs of patellar instability will need corrective surgery before or in conjunction with patellofemoral arthroplasty. A positive patellar apprehension test and the J sign are indicative of patellar instability, malalignment, or muscle imbalances. A positive J sign is visible when the knee is actively extended, and lateral patellar subluxation occurs in the terminal 20° of extension. Excessive femoral internal torsion or tibial external torsion can lead to malalignment as well. A planovalgus foot can be associated with patellar maltracking, and symptoms may resolve with orthotic treatment alone.
Palpation of the patella during tracking can help to localize pain. Pain elicited with the patellar grind or patellar tap test is indicative of patellofemoral pathology. Any tenderness to palpation at the medial or lateral joint lines usually indicates tibiofemoral involvement, a contraindication to patellofemoral arthroplasty. Although anterior or posterior cruciate ligament insufficiency is not a strict contraindication to patellofemoral arthroplasty, it is important to evaluate all knee ligaments and consider reconstruction before patellofemoral arthroplasty to avoid progressive tibiofemoral arthritis caused by instability.
The neurovascular status, skin integrity and previous scars, hip motion, and the lumbar spine should always be evaluated. Referred pain from the hip to the knee region, as well as L3-L4 nerve root radiculopathy, can produce anterior knee pain and should be considered during the examination.
Preoperative Imaging
Weight-bearing anteroposterior and lateral radiographs should be made to avoid underestimation of tibiofemoral abnormality. Lateral radiographs allow assessment of the patella, and occasionally arthritic patellofemoral changes are visible (Fig. 1). Patella baja should be corrected before patellofemoral arthroplasty18. Standing posteroanterior 45° flexion radiographs (Rosenberg views) are used to assess the extent of abnormality of the posterior femoral condyle. Axial radiographs demonstrate the extent of patellofemoral arthritis, trochlear dysplasia, and patellar tilt or subluxation. Full-length standing radiographs are useful for evaluating mechanical alignment of the entire limb (Fig. 2).
Computed tomography is used to evaluate posttraumatic osseous architecture, rotational abnormalities, and trochlear dysplasia, but has little role in assessing patellofemoral arthritis. Magnetic resonance imaging (MRI) scans with use of delayed gadolinium-enhanced imaging for cartilage (dGEMRIC) and T1rho are being studied and may prove useful in assessing articular cartilage19,20. Bone scans help to determine the extent of abnormality in the medial and lateral compartments of the knee.
Images from any prior arthroscopic procedure should be reviewed. There are no published data, as far as we know, to support routine arthroscopic examination before patellofemoral arthroplasty, but, if the surgeon believes he or she can better assess the lateral compartment, then it may be advantageous.
Early problems usually stem from implant or technique-related issues and include malalignment and implant malposition leading to catching, instability, and maltracking. Late problems are loosening and/or wear of the patellar component, loosening of the trochlear component, and development of tibiofemoral disease.
Early implant designs led to problems with patellar tracking and instability. Newer designs are more accommodating and have reduced these problems. Malposition of the implant also can cause patellar catching and instability. Placing the implant in flexion can lead to catching of the patellar component on initiation of flexion. Malrotation of the trochlea or a laterally placed patellar component can cause subluxation or dislocation with recurrent instability. A trochlear component that is too large may irritate the peripatellar retinaculum.
Some studies have described the need for manipulation with the patient under anesthesia in the early postoperative period, for 3% to 14% of patients3,14,17, and noted that it may be required to achieve 90° of flexion by six weeks postoperatively. Prior surgery may have an effect on the need for manipulation under anesthesia; however, it also can result from overstuffing of the patellofemoral joint. Measuring the patellar thickness before resection and restoring it should reduce the chance for overstuffing.
Loosening and/or wear of the patellar component in 4% of patients was reported by van Jonbergen et al.17. These patients may be candidates for revision patellofemoral arthroplasty rather than conversion to total knee arthroplasty34. Hendrix et al.34 reported mixed results in a small series (fourteen knees) in which a failed first-generation inlay patellofemoral replacement was revised to a second-generation onlay patellofemoral arthroplasty. With long-term outcome data up to fifteen to twenty years now available, the development of femorotibial osteoarthritis has been determined to be the most common reason for failure and conversion to total knee arthroplasty. Conversion rates of one in five have been reported after an average of seven to sixteen years14,31. Tibiofemoral arthritis was observed in 45% of knees and resulted in conversion to total knee arthroplasty in 13% in the study by van Jonbergen et al.17. It has been suggested that patients presenting with idiopathic patellofemoral arthritis may be more prone to progression to generalized tibiofemoral arthritis than patients with other types of arthritis, and thus caution should be used when considering these patients for patellofemoral arthroplasty25. However, van Jonbergen et al. reported that primary diagnosis and age did not significantly affect the conversion rate17.
Van Jonbergen et al.22 reported the results of conversion of fourteen patellofemoral replacements to total knee arthroplasties in thirteen patients (average age, sixty-seven; range, fifty to seventy-seven) because of tibiofemoral arthritis. At a mean follow-up of 5.7 years (range, two to thirteen years), there were no significant differences in Knee Society Scores and Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) scores when this group of patients was compared with a matched cohort. Three knees in three patients required manipulation under anesthesia, and two of those knees also required manipulation after the previous patellofemoral arthroplasty. They concluded that patellofemoral arthroplasty does not have a negative effect on the outcome of total knee arthroplasty22. On the basis of their experience in a small group of twelve patients, Lonner et al.35 agreed, concluding that the results of total knee arthroplasty do not appear to be compromised after revision of a failed patellofemoral component.
The current generation of patellofemoral arthroplasty implant designs, when used in properly selected patients on the basis of clear history, physical, and radiographic criteria, provides a sound option for the treatment of isolated osteoarthritis of the patellofemoral joint. Determining which patients may not be good candidates is difficult but is essential to ensure long-term survivorship and patient satisfaction. Correcting any patellar maltracking prior to or at the time of patellofemoral arthroplasty is mandatory, and patellofemoral arthroplasty should never be used alone to treat patellar instability.
Disclosure: None of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support of any aspect of this work. One or more of the authors, or his institution, has had a financial relationship, in the thirty-six months prior to submission of this work, with an entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. No author has had any other relationships, or has engaged in any other activities, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article.