Patella instability in children and adolescents

Abstract

Patellar instabilities are the most common knee pathologies during growth. Congenital dislocations are rare. Extensive, early soft tissue releases relocate the extensor mechanism and may enable normal development of the femoro-patellar anatomy.Conservative management is the preferred strategy after a 'first-time' traumatic dislocation. In cases with concomitant anatomical predisposing factors such as trochlear dysplasia, malalignment, malrotation or ligamentous laxity, surgical reconstruction must be considered. The same applies to recurrent dislocations with pain, a sense of instability or re-dislocations which may also lead to functional compensatory mechanisms (quadriceps-avoiding gait in knee extension) or cartilaginous lesions with subsequent patello-femoral osteoarthritis. The decision-making process guiding surgical re-alignment includes analysis with standard radiographs and MRI of the trochlear groove, joint cartilage and medial patello-femoral ligament (MPFL). Careful evaluation of dynamic and static stabilisers is essential: the medial patello-femoral ligament provides stability during the first 20° of flexion, and the trochlear groove thereafter.Excessive femoral anteversion, general ligamentous laxity with increased femoro-tibial rotation, patella alta and increased distance between the tibial tuberosity and the trochlear groove must also be taken into account and surgically corrected.In cases with ongoing dislocations during skeletal immaturity, soft tissue procedures must suffice: reconstruction of the medial patello-femoral ligament as a standalone procedure or in conjuction with more complex distal realignment of the quadriceps mechanism may lead to a permanent stable result, or at least buys time until a definitive bony procedure is performed. Cite this article: Hasler CC, Studer D. Patella instability in children and adolescents. EFORT Open Rev 2016;1:160-166. DOI: 10.1302/2058-5241.1.000018.

Keywords: dysplasia; medial patellofemoral ligament; patellar dislocation (habitual; recurrent); trochlea.

Fig. 1

14-year-old boy with acute first-time dislocation during soccer play. The MRI shows a knee effusion (haemarthrosis marked with asterisk), a shallow trochlear groove (dotted line) and an avulsion fracture of the MPFL at its patellar insertion (arrow).

Fig. 2

18-year-old boy with Down syndrome and recurrent painful dislocations of his left patella. Apart from syndrome-associated ligamentous laxity as a major predisposing factor, he displays significant trochlear dysplasia: 1) positive crossing sign (trochlear line crosses anterior femoral cortex line); 2) osseous bump (spur) at the upper border of the trochlea which misguides the patella into a lateral position when the patient is flexing his knee.

Fig. 3

12-year-old boy with chronic habitual bilateral patellar dislocations. A concomitant predisposing factor was a moderate valgus deformity, young age and a trochlear dysplasia of both knees (a). The lateral view of the left knee shows the difficulty to objectify the patella position (alta, normal or baja): the growth plate with its anterior tongue (white line) has not ossified yet. Hence, there is no distinct insertion area of the patellar ligament (dotted line) and no clear osseous reference to assess the Insall index as in skeletally mature patients. The Caton-Deschamps ratio (A/B) is more appropriate but still difficult to assess in a patella which is subluxed (b). Moreover, longstanding lateralisation of the extensor mechanism led to a knee flexion contracture which omits a standard lateral radiograph.