Evaluation of the Mechanical Failure of Noninvasive Expandable Prostheses: A Retrospective Service Evaluation of a UK Health Service.

Abstract

BACKGROUND: Noninvasive expandable prostheses (NIEPs) have revolutionized limb-salvage surgery for pediatric bone tumor patients by enabling limb lengthening without additional surgeries. Despite increasing use, evidence regarding predictors of mechanical failure remains limited. This service evaluation of NIEPs at the Royal National Orthopaedic Hospital (RNOH), United Kingdom investigated whether mechanical failure rates differ by anatomic location of Juvenile Tumour System (JTS) prostheses and whether implant location and lengthening factors influence failure risk. METHODS: A retrospective data analysis of 140 JTS prostheses implanted at the Royal National Orthopaedic Hospital (2008 to 2024) was performed. Five implant types were examined: distal femoral (DFR, n=86), proximal tibial (PTR, n=26), proximal femoral (PFR, n=21), total femoral (TFR, n=6), and femoral diaphyseal replacements (FDR, n=1). Mechanical failure was defined as any instance of the implant lengthening mechanism stopping. Kaplan-Meier survival analysis and Cox regression were used to identify predictors of mechanical failure. RESULTS: The overall mechanical failure rate was 37.9%. Total femoral (HR=11.4, 95% CI: 2.28-56.77, P=0.003) and femoral diaphyseal (HR=31.7, 95% CI: 2.86-352.39, P=0.005) replacements demonstrated significantly higher failure risks compared with distal femoral. Proximal tibial replacements showed better survival (median 31 mo) than distal femoral replacements (24 mo). While univariate analysis showed patients with mechanical failure underwent more lengthening sessions (P<0.001) and achieved greater total lengthening (P=0.002), these associations were not significant in multivariable analysis. CONCLUSION: These findings show that anatomic location significantly influences mechanical failure risk in NIEPs, with TFR and FDR implants showing substantially higher failure rates. These findings address a key knowledge gap at the RNOH and can inform their surgical planning, postoperative monitoring, and rehabilitation strategies, particularly for high-risk implant types. CLINICAL RELEVANCE: The results are directly relevant to surgical decision-making and patient counseling at the Royal National Orthopaedic Hospital and similar centers. Tailored rehabilitation protocols, such as modified weight-bearing progressions, gait training, and enhanced biomechanical assessments, may reduce failure risk in high-risk implants (TFR, FDR). In addition, lengthening strategies should also be carefully considered during planning for these implant types.