USE CAUTION WHEN ASSESSING PRE-OPERATIVE LEG LENGTH DISCREPANCY IN PEDIATRIC PATIENTS WITH ANTERIOR CRUCIATE LIGAMENT INJURIES

BACKGROUND: Pre- and post-operative standing hip to ankle radiography is critical for monitoring potential post-operative growth arrest and resultant length and angular deformities after pediatric anterior cruciate ligament (ACL) reconstruction. During acquisition of pre-operative standing alignment radiographs, it is possible that patients are lacking full extension, not weight bearing comfortably, or leaning resulting in inaccurate measurements. PURPOSE: This study aims to assess both pre- and post-operative radiographic measurements to assess if the standing pre-operative x-ray is a accurate and reliable source for baseline measurements. METHODS: We retrospectively reviewed prospectively collected pre-operative and first post-operative full-length hip-to-ankle radiographs in a cohort of skeletally immature athletes who presented with an acute ACL injury and underwent subsequent surgical reconstruction. Initially, leg length discrepancy for 25 patients was measured by 3 orthopedic surgeons (top of femoral head to center of tibial plafond). The intraclass correlation was almost perfect (ICC (2,1) = .996) therefore, 1 surgeon measured the remaining 94 radiographs. Measurements for both the injured and uninjured legs were obtained for comparison and surgeons were blinded to the injured side. RESULTS: A total of 119 pediatric patients (mean age 13.4, range 7-14 years) were included (83 males and 36 females). Patient were categorized as either having ≥5mm, ≥10mm, or ≥15mm LLD on pre-operative standing x-ray. Sixty-two patients (52%) were found to have a pre-operative LLD ≥ 5mm. Forty-one (66%) of these patients tore their ACL on the limb measuring shorter. At 6 month post-operative standing x-ray, 35 patients (56%) resolved to ≤5mm LLD. Eighteen patients had a pre-operative LLD of ≥ 10mm. At 6 month post-operative standing x-ray, 13 (72%) patients resolved to ≤5mm LLD. Five patients had a pre-operative LLD of ≥ 15mm. At 6 month post-operative standing x-ray, 4 (80%) resolved ≤5mm. All patients with a pre-operative LLD of ≥ 13mm had sustained an ACL injury on the limb measuring shorter CONCLUSION: Of the pediatric ACL patients initially presenting with a pre-operative LLD of ≥ 10mm, 72% demonstrated apparent correction of their LLD on their 6 month standing x-ray. This high rate of LLD pre-operatively but not post operatively calls into question the accuracy of pre-operative standing alignment radiographs for patients after an ACL tear. Surgeons and radiology technicians should be aware of injured patients potentially lacking full extension, leaning, or not weight bearing comfortably, and should consider delaying preoperative radiographic length and alignment analysis until after the patient is able to fully straighten the injured knee and weight bear comfortably.