A 19-year-old Japanese female presented with persistent right knee pain after an atraumatic episode of patellar dislocation during walking. Patient had a previous history of right recurrent patella instability with an onset at the age of 8 years due to minor trauma. Mild genu valgum and genu recrurvatum were observed, along with a positive patellar apprehension sign, bilaterally. Flatfeet and sandal gaps were apparent (Fig. 1a). No facial anomalies or nail deformities were observed. Plain radiographs revealed bilateral small patella, hypoplasia of the tibial tuberosity (Fig. 1b), and elongated femoral necks with hypoplastic lesser trochanters (Fig. 1c). Computed tomography images of the right knee revealed a patellar subluxation associated with hypoplasia of the femoral trochlea groove, where the tubercle-trochlear groove (TT-TG) distance was 22 mm (Fig. 1d).
After informed consent was obtained from the patient and her family members, mutation analysis of the coding regions in the TBX4 gene was performed. Genomic DNAs purified from peripheral blood of the patient and saliva of the family members (i.e. parents and sister) were assessed by direct sequencing using the QIAamp DNA Blood Midi kit (Qiagen, Inc., Valencia, CA). A novel heterozygous frameshift mutation (p.L39PfsX35) in exon 1 of the TBX4 gene, was found in the present case (Fig. 2a). Analyses of the parents and sister revealed that the TBX4 variant was derived from her father, who had neither medical history of skeletal dysplasia nor restricted activities of daily living (Fig. 2b).
Since conservative management (i.e. rest, analgesics, physiotherapy, brace, etc.) was not successful, surgical treatment was indicated for worsening pain in the right knee due to RPD. Under general anesthesia, a lateral retinacular release was performed arthroscopically, followed by open medial patellofemoral ligament (MPFL) reconstruction. The semitendinous tendon was harvested from the ipsilateral knee, and both ends of the tendon graft were then prepared with a running baseball stitch using a No.2 FiberWire (Arthrex, Naples, FL). The femoral bone tunnel was established according to the method reported by Schottle et al., where one end of the graft was passed through and fixed with an EndoButton (Smith & Nephew, London, England). Due to the small size of patella, transosseous graft fixation was not attempted for the patellar side considering a risk of fracture. Therefore, the other end of the graft was subperiosteally passed through one-third of the proximo-medial patella, and fixed onto the lateral aspect of the patella using braided nylon sutures with the knee in 90  of flexion. Distal realignment osteotomy (e.g. Elmslie-Trillat procedure) was not indicated because of the hypoplastic tibial tuberosity with no prominence. Postoperative course was uneventful. No recurrent pain or instability have been reported and maintained reduction of the patella has been radiographically confirmed at one year following surgery (Fig. 3a, Fig. 3b).