- Careful evaluation is key in attributing knee pain to patellofemoral cartilage lesions-that is, in making a "diagnosis by exclusion".
- Initial treatment is nonoperative management focused on weight loss and extensive "core-to-floor" rehabilitation.
- Optimization of anatomy and biomechanics is crucial.
- Factors important in surgical decision-making incude defect location and size, subchondral bone status, unipolar vs bipolar lesions, and previous cartilage procedure.
- The most commonly used surgical procedures-autologous chondrocyte implantation, osteochondral autograft transfer, and osteochondral allograft-have demonstrated improved intermediate-term outcomes.
Patellofemoral (PF) pain is often a component of more general anterior knee pain. One source of PF pain is chondral lesions. As these lesions are commonly seen on magnetic resonance imaging (MRI) and during arthroscopy, it is necessary to differentiate incidental and symptomatic lesions. 1 In addition, the correlation between symptoms and lesion presence and severity is poor.
PF pain is multifactorial (structural lesions, malalignment, deconditioning, muscle imbalance and overuse) and can coexist with other lesions in the knee (ligament tears, meniscal injuries, and cartilage lesions in other compartments). Therefore, careful evaluation is key in attributing knee pain to PF cartilage lesions—that is, in making a "diagnosis by exclusion."
From the start, it must be appreciated that the vast majority of patients will not require surgery, and many who require surgery for pain will not require cartilage restoration. One key to success with PF patients is a good working relationship with an experienced physical therapist.
The primary causes of PF cartilage lesions are patellar instability, chronic maltracking without instability, direct trauma, repetitive microtrauma, and idiopathic.
Patients with patellar instability often present with underlying anatomical risk factors (eg, trochlear dysplasia, increased Q-angle/tibial tubercle-trochlear groove [TT-TG] distance, patella alta, and unbalanced medial and lateral soft tissues 2). These factors should be addressed before surgery.
Patellar instability can cause cartilage damage during the dislocation event or by chronic subluxation. Cartilage becomes damaged in up to 96% of patellar dislocations. 3 Most commonly, the damage consists of fissuring and/or fibrillation, but chondral and osteochondral fractures can occur as well. During dislocation, the medial patella strikes the lateral aspect of the femur, and, as the knee collapses into flexion, the lateral aspect of the proximal lateral femoral condyle (weight-bearing area) can sustain damage. In the patella, typically the injury is distal-medial (occasionally crossing the median ridge). A shear lesion may involve the chondral surface or be osteochondral ( Figure 1A ).In an osteochondral lesion, the area of cartilage damage is often larger than the bony fragment indicates ( Figure 1A ), and even small fractures visible on radiographs can portend extensive cartilage damage. In addition, isolated cartilage flaps can occur; if suspected, they should be assessed with MRI. The extent of cartilage damage is related to the magnitude of energy required to cause the dislocation and/or to the frequency of events. In more normal anatomy, more energy is required to provoke a dislocation, and damage to articular cartilage is greater. In recurrent patellar dislocation, each event can cause additional injury, and the size of the lesion tends to increase with the number of dislocations. 4 Patellar dislocation can result in chronic patellar subluxation, or dislocations that often lead to recurrent or chronic patellar instability. With recurrent instability, the medial patellar facet becomes damaged as it displaces out of the trochlea during subluxation and dislocation events. With lateral patellar maltracking, the contact area is reduced. With overall similar PF forces, a smaller contact area results in increased point loading, thus increasing stress and promoting cartilage wear.
Chronic Maltracking Without Instability
Chronic maltracking is usually related to anatomical abnormalities, which include the same factors that can cause patellar instability. A common combination is trochlear dysplasia, increased TT-TG or TT-posterior cruciate ligament distance, and lateral soft-tissue contracture. These are often seen in PF joints that progress to lateral PF arthritis. As lateral PF arthritis progresses, lateral soft-tissue contracture worsens, compounding symptoms of laterally based pain. With respect to cartilage repair, these joints can be treated if recognized early; however, once osteoarthritis is fully established in the joint, facetectomy or PF replacement may be necessary.
With the knee in flexion during a direct trauma over the patella (eg, fall or dashboard trauma), all zones of cartilage and subchondral bone in both patella and trochlea can be injured, leading to macrostructural damage, chondral/osteochondral fracture, or, with a subcritical force, microstructural damage and chondrocyte death, subsequently causing cartilage degeneration (cartilage may look normal initially; the matrix takes months to years to