Episodic patellar dislocation (EPD) is a disabling problem for some young and active people.

The primary factors predisposing to patellar instability are injury to the medial patellofemoral ligament (MPFL),¹ trochlear dysplasia,²,³ increased tibial tubercle-trochlear groove (TT-TG) distance,⁴ and patella alta.⁵,⁶,⁷,⁸

Other factors that contribute to patellar instability include excessive femoral anteversion, genu recurvatum, or genu valgum.

Patella alta has been noted to be present in nearly one-quarter of patients with EPD but in only 3% of controls.⁹ It is a risk factor not only for patellar instability but also for failure of nonoperative treatment¹⁰ or for failure of isolated MPFL reconstruction¹¹ in patients with EPD.

This morphologic abnormality (patella alta) precludes the patella from engaging in to the trochlea. From a patellofemoral engagement perspective, patella alta could be caused either by an excessive proximal position of the patella or by a short trochlea.

The association between patella alta and EPD is likely multifactorial. Patella alta would lead to a delay in patellofemoral contact and would require more knee flexion for patellofemoral engagement. This effect, in association with dysplasia of the trochlea, would lead to decreased resistance to lateral translation of the patella.⁸,¹²

In the presence of patella alta, the contact forces between the patellar cartilage and the trochlea would be concentrated on a smaller surface at the distal part of the patella when the knee is near extension. This could potentially increase the rate of patellofemoral osteoarthritis.¹³,¹⁴,¹⁵

It has been suggested that increased patellar tendon length may be the culprit because an increased length would allow for a pathologic increase in coronal plane motion of the patella.⁶ It has been noted that in patients with patella alta and EPD, the high riding patella could be because of an increased patellar tendon length (>52 mm) rather than an isolated proximal position of the tibial tubercle.⁶

It may thus be desirable to address the length of the patellar tendon itself in addition to tibial tubercle distalization. It has been shown that by reducing the patellar tendon length by 10%, an increase in patellofemoral contact area by 15% to 18% can be achieved without increasing patellofemoral stresses.¹⁶

A patellar tendon tenodesis (patellar tenodesis) would be indicated in association with a tibial tubercle distalization when the patellar tendon is too long, that is, more than 52 mm in length.⁶ By these criteria, almost 20% of patients who require a tibial tubercle distalization would also need a simultaneous patellar tenodesis.

Besides patella alta, the presence of other anatomic risk factors would require concomitant procedures to address them, such as MPFL reconstruction or trochleoplasty.

An isolated MPFL reconstruction in the presence of patella alta would be usually insufficient to manage patellar instability. Berard et al reported femoral tunnel widening after MPFL reconstruction, in patients with EPD and patella alta that were treated with an isolated MPFL reconstruction.¹⁷ The most likely explanation for tunnel widening in patella alta would be related to graft anisometry, which may cause graft micromotion and subsequent graft stress.

Neyret et al⁶ have suggested that the excess patellar tendon length may be a critical factor in patellar instability. An isolated distalization, with a long patellar tendon (>52 mm), may not stabilize the patella. The risk with a long patellar tendon is that the patella makes a “wiper” effect, with side-to-side movement of the patella on the excessively long patellar tendon. Increased mobility of only 4 mm of side-to-side movement has been associated with subjective feelings of patellar instability.¹⁸ Patellar tenodesis increase the contact area between the patella and the trochlea.

Yin et al¹⁹ analyzed patellofemoral contact mechanics using finite-element models of isolated tibial tubercle distalization and tibial tubercle distalization with patellar tenodesis. When averaged across distalization distances (from 4 to 20 mm), distalization and distalization + tenodesis both reduced cartilage stress, increased contact area, and reduced contact force compared to the baseline value. However, distalization led to greater decrease in cartilage stress and contact forces compared to distalization + tenodesis.

Table 17.1 lists indications and contraindications for patellar tenodesis.

EPD is diagnosed in patients with a history of at least one true patellar dislocation, symptoms of recurrent instability (either recurrent dislocation or subjective feelings of instability limiting function), and a positive patellar apprehension test.

Radiographic evaluation should include weight-bearing anteroposterior and lateral views of the knee and an axial view of the patella with the knee in 30° of flexion.

The height of the patella can be quantified by numerous indices, which are classified into two groups: “femoral” (or “trochlear”) indices and “tibial” indices. Femoral indices are dependent on the angle of knee flexion because the patella would move relative to the femur, compared to the tibial indices where the patella would be static. To accurately assess the patellar height, it is preferable to use both—a “tibial” index and a “femoral” index—especially when the femoral trochlear groove is dysplastic.

The most commonly used tibial indices with a native knee are the Caton-Deschamps index²⁰ and the Insall-Salvati ratio.²¹

The Caton-Deschamps index (Figure 17.1) is the ratio of the distance from the lower edge of the articular surface of the patella to the anterosuperior angle of the tibia outline (AT) to the length of the articular surface of the patella (AP). An AT/AP ratio of 0.6 and smaller would indicate patella baja or infera. A ratio of 1.2 and greater would indicate patella alta. An advantage of this index is that it could be used for planning the amount of correction needed for a tibial tubercle transfer.

The Insall-Salvati ratio (Figure 17.2) is the ratio of the length of the patellar tendon (LT) to the longest sagittal diameter of the patella (LP). Normally, the LT/LP ratio is 1. A ratio less than 0.8 would indicate patella baja or infera. A ratio greater than 1.2 would indicate patella alta. A modified Insall-Salvati ratio has been described, which would use the patellar articular surface instead of the longest sagittal diameter.