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BLOG: Surgical correction for patellar instability and genu valgum in skeletally immature patients
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Medial patellofemoral ligament reconstruction has been popularized as the surgical treatment of choice for most patients with patellar instability. Despite encouraging clinical results, there is a 10% to 20% failure rate after this reconstruction. Though some of these failures have been related to technical errors, others have been attributed to underlying anatomical risk factors that contribute to patellar instability. Such risk factors include trochlear dysplasia, patellar height, tibial tubercle lateralization and lower extremity malalignment. In addition to this reconstruction, there is considerable interest in evaluation and possible correction of these risk factors to achieve better outcomes after surgical treatment of patellar instability.
Patellar instability in skeletally immature patients continues to be a challenging subset to treat. The presence of open physis and skeletal age younger than 14 years have been identified as individual risk factors for patellar instability. The failure rate after medial patellofemoral ligament (MPFL) reconstruction is higher in skeletally immature patients than in adults. Presence of physis, physiologic joint laxity, underlying malformations or syndromic associations could further complicate the management of instability in these patients. Considering these factors, it is important to identify, analyze and possibly treat modifiable risk factors when addressing patellar instability in skeletally immature patients.
Adult vs pediatric patients
For frontal plane lower extremity alignment, the normal anatomical or diaphyseal tibiofemoral angle is about 6° valgus. An increase in this angle results in genu valgum (or knock knees). Genu valgum has been associated with altered biomechanical forces acting on the lateral tibiofemoral and patellofemoral joint. It increases the resultant lateral force vector that causes the patella to translate laterally, causing either increased lateral patellofemoral joint contact pressure or lateral patellar instability. Thus, in presence of genu valgum, there is a concern that the MPFL graft may experience increased tensile forces that can lead to graft stretch and failure. Hence, as a part of management of symptomatic patellar instability, correction of excessive genu valgum is desirable.
Correction of genu valgum in an adult would require varus-producing osteotomy. However, in skeletally immature patients with open physis, genu valgum can be corrected by growth modulation, ie, medial hemi-epiphysiodesis of distal femur. Since growth modulation is a less invasive and less morbid procedure compared to an osteotomy, the threshold for correction of genu valgum is relatively lower in skeletally immature patients. The authors have recently completed a review of 10 skeletally immature patients treated with simultaneous MPFL reconstruction and medial hemi-epiphysiodesis of distal femur for management of patellar instability in presence of genu valgum. At latest follow-up, all patients had successful correction of genu valgum and satisfactory results of MPFL reconstruction.
Technical considerations
There are some technical considerations during MPFL reconstruction when combined with growth modulation. When medial hemi-epiphysiodesis of distal femur is performed using the current popular technique of tension-band plate across the medial distal femoral physis, there is a potential for impingement of the MPFL graft and tension-band plate. If the plate is placed over the MPFL graft, it would shorten the effective graft and if the graft is placed over the plate, it could cause graft attrition. Bachman and colleagues showed the native MPFL was either transected or damaged following placement of tension-band plate in six of eight cadaver knees. Métaizeau and colleagues had popularized the technique of percutaneous transphyseal screw placement as a method for epiphysiodesis. Since the transphyseal screw trajectory is anterior to the MPFL femoral tunnel, it could achieve correction of genu valgum without interference with MPFL graft (Figure). Preliminary results do not show any deleterious effect of gradual correction of genu valgum on stability of MPFL reconstruction. Genu valgum correction by guided growth during MPFL reconstruction can obviate the need for varus-producing osteotomy later.
Figure. Since the transphyseal screw trajectory is anterior to the MPFL femoral tunnel, it could achieve correction of genu valgum without interference with MPFL graft.
Shital N. Parikh, MD, FACS, is an associate professor in the Department of Orthopaedic Surgery at Cincinnati Children’s Hospital and University of Cincinnati. He is a board-certified pediatric orthopedic surgeon specializing in pediatric sports medicine and patellofemoral disorders. He can be reached at shital.parikh@cchmc.org.
References:
Bachmann M, et al. Arch Orthop Trauma Surg. 2014;doi:10.1007/s00402-014-2032-6.
Métaizeau JP, et al. J Pediatr Orthop. 1998:18(3):363-369.
Diclosure: Parikh reports no conflicts of interest.