Patella alta, patellar instability: Complex interplay between patellar height, instability and other risk factors

During normal patellar tracking, the patella engages in the trochlea at around 30° of knee flexion and remains stable then after due to the bony constraints of the trochlea. Most patellar instability is seen during this early arc (0° to 30°) of knee flexion as the patella is not engaged into the trochlea. Thus, one can understand that patella alta or proximally located patella would be an important risk factor for patellar instability as the patella now has more distance to travel before it engages the trochlea, thus exposing it to instability and maltracking.

Although the relationship of patellar height and patellar instability could be easy to conceptualize in isolation, there is a complex interplay between patellar height, patellar instability and various other instability risk factors and these relationships between different risk factors and their combined effects are not fully understood. Also, the measurement of patellar height and management of patella alta are not well defined.

Patellar height

Patellar height can be measured on conventional lateral radiographs by several published indices or ratios, including Insall-Salvati ratio, Caton-Deschamps index and Blackburne-Peel ratio. The issue is the relationship between these different indices is not established and in some cases cannot be established as each index measures from different radiographic landmarks. Thus, it is possible that the patellar height may be normal when measured by one index and abnormal when measured by another index. If meaningful comparisons have to be drawn from any studies, it is of utmost importance to standardize measurements, before management principles are considered. Recently, MRI has been used to assess patellar-trochlear engagement on axial and sagittal sections. These indices appear to be more sensitive than radiographic measurement of patellar height as inadequate patellar engagement were recorded despite normal radiographic patellar height. These measurements, however, need further validation.

Shital N. Parikh

Besides patella alta, there are several other well-established risk factors that contribute to patellar instability including trochlear dysplasia, laterally placed tibial tubercle, patellar tilt, hyperlaxity and lower limb malalignment. While some surgeons deem it necessary to address several or all risk factors during surgery for patellar stabilization, others perform an isolated MPFL reconstruction knowingly ignoring some or most of the risk factors.

There is low-level evidence to support both treatment philosophies. For example, MPFL reconstruction and trochleoplasty is recommended in patients with patellar instability and trochlear dysplasia. However, isolated MPFL reconstruction has shown promising results even in the presence of trochlear dysplasia. It is possible that isolated MPFL reconstruction can compensate for lower degrees or lower number of risk factors and it might fail beyond a certain threshold. Though attempts have been made to quantify such thresholds for each individual risk factor, a composite scoring system which would consider all established risk factors to help guide comprehensive management for patellar instability has not been established.

Management of patella alta

Management of patella alta is highly controversial. Some authors recommend tibial tubercle distalization or patellar tendon tenodesis to address increased patellar height during patellar stabilization. Two recent studies, however, have reported that isolated MPFL reconstruction can decrease patellar height and hence tibial tubercle distalization may be seldom necessary. The reason for decrease in patellar height is the vector of MPFL which is directed not straight medially, but medially and inferiorly by about 15° from patella to medial femoral condyle. Thus in patients with patellar instability and borderline patella alta, an isolated MPFL reconstruction may suffice to stabilize the patella and normalize patellar height. It also means that higher the patella, more would be the strain on the MPFL graft to pull it inferiorly.

This has been shown in a recent study, where there was dilatation of the femoral tunnel when MPFL reconstruction was used in presence of patella alta. The clinical implications of these radiographic findings are not known yet, but there may be role of tibial tubercle distalization in some patients with severe patella alta. Although tibial tubercle distalization has been proposed as an adjunctive procedure during patellar stabilization in patients with patella alta, there is paucity of published clinical results of this procedure.

A recent systematic review of treatment of patella alta in skeletally mature patients with patellar instability could identify only five retrospective studies between 1966 and 2012. The associated procedures performed at the time of tibial tubercle distalization in these studies included vastus medialis advancement (three studies) and MPFL reconstruction (one study). Although all five studies demonstrated radiographic normalization of patellar height and low risk of recurrent patellar dislocation, there was a 26% risk of continued apprehension and several patients reported persistent symptoms. None of the studies had preoperative patient reported outcome scores or a control group and only three studies reported postoperative outcome scores.

Complications

Complications of patellar distalization included tibial tubercle nonunion, proximal tibial fracture, infection, deep vein thrombosis, radiographic over-correction and patellofemoral arthritis. In another study on athletes with patellar instability, the authors reported satisfactory clinical results of lateral release and modified Fulkerson osteotomy, where anteromedialization was combined with distalization of the tibial tubercle to address concomitant patella alta.

Several biomechanical and imaging studies have focused on the deleterious influence of patella alta on patellofemoral contact force, contact area and contact pressure, but there is no study on the influence of tibial tubercle distalization on these parameters. It is assumed that the radiographic decrease in patellar height after tibial tubercle distalization would restore patellar stability and normalize patellofemoral joint biomechanics. However, unexpected alteration in joint biomechanics, effect of over-correction and the cumulative effect of associated procedures (e.g., MPFL reconstruction) may lead to less than desired biomechanical and clinical outcomes following patella alta correction. Until clinical and biomechanical studies further clarify the indications and effects of tibial tubercle distalization on patellofemoral joint, such procedure should be undertaken with much caution.

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Shital N. Parikh, MD, FACS, is an associate professor of orthopedic surgery, Cincinnati Children’s Hospital Medical Center, University of Cincinnati School of Medicine, Cincinnati. He can be emailed at:Shital.Parikh@cchmc.org.

Disclosure: Parikh has no relevant financial disclosures.