Chronic lateral dislocation of the patella occurring primarily during knee flexion is rare and frequently underdiagnosed. It is often linked to trochlear dysplasia and, in some cases, to a prominent anterior femoral bump that disrupts patellar engagement during flexion ^[1]. In such cases, static soft tissue restraints, particularly the medial patellofemoral ligament (MPFL), may be insufficient or stretched ^[2]. This case demonstrates the successful treatment of such a complex instability using a combined bony and soft tissue approach: distal femoral anterior subtraction osteotomy per Pujol’s technique ^[1] and MPFL reconstruction.

A 43-year-old woman originally from Ukraine presented with chronic instability from child of the right patella during flexion at 70°. No prior trauma or surgery was reported. The dislocation was reproducible during squatting or climbing stairs, without instability in extension.

Clinical findings:

• Positive J-sign
• Patella stable in extension, lateral dislocation at 60–90° flexion
• Positive Smillie test, with increased lateral translation in flexion
• Mild quadriceps atrophy, no hyperlaxity elsewhere
• Kujala score 55
• IKDC score 32
• KOOS score 55.

Imaging:

• X-ray (Fig. 1), computed tomography (Fig. 2) and magnetic resonance imaging showed Dejour type D trochlear dysplasia
• Anterior femoral bump visible in lateral view (9 mm)
• TT-TG: 11 mm
• Normal femoral version and tibial torsion.

Surgical technique

The patient underwent combined bony and soft tissue surgery:

1. Distal femoral anterior closing-wedge osteotomy (Fig. 3)

• Via medial parapatellar approach
• Marking the osteotomies on the bone with a dermographic pen, according to the pre-operative planning
• Performing the anteroposterior cut first using a reciprocal saw
• Making the posterior cut, strictly parallel to the frontal plane of the femur:

Note: The cut is stopped approximately 5 mm from the sulcus terminalis to preserve the distal osteochondral hinge.

• Performing the anterior slanting cut to connect the two previous cuts
• Removing the bone wedge (proximally based)
• Progressive closure of the osteotomy by applying digital pressure on the trochlea
• Fixation of the osteotomy using 3.5 mm cancellous screws

Note: Positioned just laterally to the cartilage surface.

2. MPFL reconstruction (Fig. 4)

• Autologous gracilis graft harvested
• Femoral tunnel positioned under fluoroscopic control at Schöttle’s point
• Patellar insertion through two anchors
• Tensioning at 30° of flexion to avoid overtightening
• Femoral fixation with screw.

Post-operative protocol:

• Knee brace in extension for 3 weeks
• Passive flexion from day 1 up to 90°
• Partial weight bearing at 3 weeks
• Active rehab with closed-chain strengthening at 6 weeks.

Outcomes

At 6 months:

• No episodes of dislocation or subluxation
• ROM: 0–130°, symmetrical with contralateral knee
• Kujala score improved from 58 to 87
• IKDC score improved from 32 to 82
• KOOS score improved from 55 to 85
• Clinical patellar tracking was centered throughout the range of motion
• Patella remained stable under manual testing (negative Smillie and J-sign)
• Radiographs showed complete bone healing and restored slope (Fig. 5).

Chronic patellar dislocation during flexion without prior trauma is often misdiagnosed. The failure of patellar engagement with the trochlea during flexion may be due to both soft tissue and bony anomalies. The femoral bump, described by Pujol and Beaufils ^[1], is an anterior overgrowth of the distal femur, which impairs patellar glide and promotes lateral dislocation under flexion stresses. While MPFL reconstruction is commonly used to address recurrent patellar instability ^[2,3], its isolated use in cases with bony deformity can lead to recurrence or graft overload ^[4]. Thus, an osteotomy that corrects the anatomical abnormality, in this case via anterior subtraction, is justified. Previous work by Dejour et al. ^[5] has shown that patellofemoral instability is multifactorial, often requiring combined surgical techniques. Our patient presented with a positive Smillie test and demonstrated a femoral bump on CT imaging, with no rotational malalignment. The use of distal femoral osteotomy to remove the bump corrected the bony conflict and restored patellar tracking. The addition of MPFL reconstruction further enhanced medial stability, particularly in the early post-operative period. Biomechanical studies have confirmed that the MPFL contributes to 50–60% of medial restraint during early flexion ^[6,7]. Positioning the femoral tunnel correctly at Schöttle’s point is critical to avoid overtensioning or failure ^[8]. Other studies have described similar combined approaches with encouraging outcomes. Wagner et al. ^[9] noted improved tracking with dual interventions in patients with complex trochlear or femoral morphology. Long-term results are encouraging, but careful patient selection and anatomical analysis remain essential ^[10].

Combined trochleoplasty and MPFL reconstruction is a useful and reliable surgical technique to improve patellofemoral stability in patients with trochlear dysplasia.

Chronic patellar dislocation during knee flexion is a rare condition, often associated with femoral dysplasia and dynamic patellar maltracking. A thorough understanding of the underlying pathology and the available treatment options is essential for effective management.