Over a period of 3 years, six consecutive patients with an indication for a lateral trochlear lengthening osteotomy were operated by two senior surgeons and included in this study. The indications for the lateral trochlea lengthening osteotomy included the presence of patellofemoral pain and/or patella instability with a short lateral condyle as quantified by the lateral condylar index (values below 93% are considered pathological)  and an absence of other clinical or radiological abnormalities to be corrected according to the Lyon ‘menu à la carte’. Namely, this procedure was indicated in patients with trochlear dysplasia Dejour type A or lack of Dejour type dysplasia, normal tibial tubercle to trochlear groove (TT-TG) distance (< 20 mm), Caton–Deschamps index (0.6–1.2) , patellar tilt (< 20°)  and lack of femoral and tibial torsional deformities (Fig. 1).
Pre- and postoperative clinical and radiological measurements
Preoperatively, patients were evaluated clinically and radiologically. Clinical evaluation included range of motion (ROM), lateral gliding test , patellar apprehension test (Fairbank’s test)  and moving patellar apprehension test (MPAT)  as well as control for torsional deformities of the tibia  and of the femur . The radiological evaluation included anteroposterior and lateral plane radiographs of the knee, skyline patellar view images and magnetic resonance images (MRI). Torsional deformities were investigated with MRI and computer tomography (CT) scans only if the patient had pathologic values on the clinical examination of the femoral and tibial torsion. The trochlea dysplasia according to the Dejour classification was documented according to the axial views in the MRI [2, 18]. Furthermore, in sagittal MRIs the Caton-Deschamps index  was measured to evaluate a possible patella alta. In axial MRIs the TT-TG distance  was measured to evaluate a possible tubercle lateralization and the patellar tilt  as a sign of muscular and ligamentous balance. The presence of a Biedert type trochlear dysplasia was evaluated according to the lateral condylar index. The measurement of this index has been previously described . Briefly, sagittal MRIs are used to draw the axis of the femur at the level where the anterior cruciate ligament is present at its entire length. The length of the most anterior and posterior aspects of the cartilaginous part of the lateral condyle is measured. The index of the anterior/posterior cartilaginous length is the lateral condylar index, expressed as a percentage (Fig. 2). Values below 93% are considered pathologic, and values below 86% are diagnostic for the presence of a short lateral condyles.
Patients were followed up 6 weeks, 12 weeks, 6, 12, and 24 months postoperatively. An additional last follow up was performed at the time of conduction of this study. The initial clinical tests were performed at each follow up. Anteroposterior and lateral views of the knee as well as axial patellar views were performed 6 weeks postoperatively. Additionally, an MRI was performed in patients having discomfort at the 12 weeks follow up. The subjective evaluation was conducted through the Kujala Anterior Knee Pain Scale , the Lysholm Knee Score and the Tegner Activity Score . Furthermore, patients were asked to subjectively evaluate the outcome of the operation according to the following options (subjective evaluation score): 5—excellent; 4—somewhat improved; 3—unchanged; 2—somewhat worse; 1—significantly worse , and asked if they would have the operation again. Lastly, isokinetic knee flexor and extensor strength tests were performed at 60°/s using an isokinetic dynamometer (Biodex System 4 Pro: Biodex Medical Systems, Shirley, NY, USA). The limb symmetry index (LSI)  (maximum torque operated side/maximum torque unaffected side × 100) of the flexors (hamstrings) and extensors (quadriceps) as well as the hamstring to quadriceps ratio (H/Q ratio)  were calculated.
The patients were operated according to the technique described by Biedert et al. . Through a short lateral parapatellar approach the length of the lateral trochlea was first observed, and the pathology confirmed. An osteotomy was performed with a chisel 5 mm posterior to the lateral cartilage of the trochlea along the anteroposterior axis and at the level of trochlear cartilage to periosteum transition in craniocaudal direction. The cartilage and periosteum were elevated proximally. Cancellous bone was harvested from the lateral femoral condyles and inserted in the osteotomy gap in a press fit technique. Allograft was used in case that the harvested cancellous bone was not sufficient. The periosteum was reattached with an absorbable Vicryl size 2 suture (Figs. 3, 4, 5). In case of the presence of a symptomatic lateral bump this was corrected as part of the lengthening procedure (Fig. 6). Figure 7 demonstrates an illustration of the technical procedure.
Since the lateral lengthening osteotomy is rarely performed as a single operation, a part of the rehabilitation may be dictated by the concomitant procedures. Postoperatively, the patients were mobilized with partial weight bearing of 15 kg for 6 weeks. A hinged knee brace with a restriction of the knee joint ROM of 0/0/60° was applied for 3 weeks and 0/0/90° for another 3 weeks. Physiotherapy was initiated at the first postoperative day and continued for 6 months postoperatively. For the first 6 weeks only limited exercises with the goal of effusion elimination, gait control and leg control restoration were performed. After week 6, continuous passive motion and active ROM exercises, closed chain quadriceps exercises, and balance and proprioception exercises were included. After week 12, sport specific exercises were added. Return to sports was allowed when full ROM and lack of discomfort were achieved.