Quantitative Gait Assessment before and after Limb Lengthening in a Patient

with Achondroplasia: A Case Report

Authors: Yusuke Yasutani, Hiroki Fujita, Takahito Inoue, Atsushi Teramoto

Abstract

Background:

Achondroplasia, a common skeletal dysplasia, results from a gain-of-function mutation in the fibroblast growth factor receptor 3 gene. It is characterized by short-limbed dwarfism and is frequently associated with skeletal deformities, including genu varum and tibial bowing, which often impair gait and limit daily function. Although limb lengthening with an external fixator is a widely utilized intervention, quantitative evaluations of its effectiveness remain limited.

Case:

A 10-year-old boy with achondroplasia presented with progressive lower limb deformity and gait disturbance. Preoperative assessments revealed pronounced genu varum, tibial bowing, and mechanical axis deviation. Bilateral tibiofibular lengthening using the Taylor Spatial Frame was performed, achieving a 6-cm increase in limb length. The external fixator was removed 1 year postoperatively. Gait rehabilitation commenced 4 months postoperatively, with full independent ambulation achieved by 6 months. Quantitative assessments using the 6-minute walk test (6MWT) and three-dimensional gait analysis (3DGA) demonstrated improvements in walking distance, cadence, and a reduction in the Physiological Cost Index (0.29 to 0.1). Preoperative 3DGA revealed significant gait deviations, which improved postoperatively, including a 9.5° reduction in the Gait Profile Score.

Discussion:

Moment analysis indicated reductions in knee flexion and varus moments, suggesting improved gait efficiency. These findings suggest that limb lengthening enhances both limb alignment and functional gait performance. The integration of 3DGA and 6MWT with conventional radiographic evaluation offers a comprehensive assessment of treatment outcomes, reinforcing the effectiveness of surgical intervention and rehabilitation in patients with achondroplasia.

INTRODUCTION

Achondroplasia is a representative skeletal dysplasia, occurring in approximately 4.6 per 100,000 live births.^1^^,^^2^^)^ It results from a gain-of-function pathogenic variant in the fibroblast growth factor receptor 3 gene.^3^^)^ Achondroplasia is characterized by short-limbed dwarfism and is frequently associated with skeletal deformities such as genu varum and tibial bowing. These deformities significantly impact patients’ activities of daily living and gait, necessitating appropriate therapeutic interventions. Limb lengthening using an external fixator is an effective approach for both increasing limb length and correcting skeletal deformities. Therefore, an objective assessment of its efficacy is crucial. In this study, limb lengthening surgery was performed on a pediatric patient with achondroplasia who presented with severe tibial bowing and genu varum. To evaluate treatment outcomes, a three-dimensional gait analysis (3DGA) was conducted both preoperatively and postoperatively. Currently, limited studies have employed preoperative and postoperative 3DGA in the context of limb lengthening for achondroplasia, and standardized quantitative evaluation criteria have yet to be established. This report provides a detailed analysis of the effectiveness of limb lengthening surgery and its impact on gait function based on 3DGA findings.

CASE

The patient was a 10-year-old boy. He was born at 39 weeks and 5 days of gestation via cesarean section after abnormal fetal rotation. The birth weight was 3315 g, and no complications were noted at birth. Limb shortening was observed on the first day of life, and genetic testing at approximately age 3 confirmed achondroplasia. At age 10, the patient presented to our department for initial consultation to address progressive lower limb deformities and associated gait abnormalities, seeking treatment including surgical intervention.

At the initial visit, the patient exhibited marked limb shortening and deformities. Figure 1 shows the clinical presentation and plain radiographs at the first consultation. The femorotibial angle (FTA) was 199° on the right and 195° on the left, and the tibial varus angle was 44° on the right and 43° on the left. The zone of mechanical axis (ZMA) was classified as Zone −3 bilaterally. Key clinical concerns included lower limb shortening, genu varum, severe tibial bowing, and gait dysfunction. A single-stage limb lengthening and gradual correction procedure was planned to address these deformities.

Preoperative planning was performed using dedicated software, and bilateral tibiofibular lengthening was conducted using a Taylor Spatial Frame (Smith and Nephew, Memphis, TN, USA). Postoperative plain radiographs are shown in Fig. 2. Bilateral lengthening of 6 cm was achieved. The post-lengthening radiographs are presented in Fig. 3. One year postoperatively, the external fixator was removed. Clinical images and radiographs following fixator removal are shown in Fig. 4. Although tibial skew deformities remained, postoperative measurements showed improvements in the FTA (right, 180°; left, 182°), tibial varus angle (right, 5°; left, 3°), and ZMA (both sides improved to Zone +1).

Postoperative rehabilitation began with walker-assisted gait training at 4 months postoperatively, followed by weight-bearing frame-assisted walking practice at 5.5 months postoperatively. Independent ambulation training began at 7 months postoperatively, and full-day independent walking was achieved by 6 months postoperatively. Implant removal was performed 1 year postoperatively. One-quarter partial weight-bearing was permitted at 3 weeks post-external fixator removal, progressing to half weight-bearing at 4 weeks, and full weight-bearing from 5 weeks post-external fixator removal.

In a preoperative assessment by a physical therapist, the following data were recorded in conjunction with a 6-minute walk test (6MWT): resting heart rate, 84 bpm; post-exercise heart rate, 102 bpm; Physiological Cost Index (PCI), 0.29; walking distance, 367 m; cadence, 124 steps/min; subjective fatigue rating, 7.5/10. At final follow-up (17 months postoperatively, 5 months post-external fixator removal), the following data were recorded in conjunction with the 6MWT: resting heart rate, 113 bpm; post-exercise heart rate, 119 bpm; PCI, 0.1; walking distance, 411 m; cadence, 132 steps/min; subjective fatigue rating, 1/10. Compared to preoperative values, multiple parameters demonstrated improvement.

This study was approved by the Institutional Review Board of the Hokkaido Medical Center for Child Health and Rehabilitation (Approval 34-58; 1 April 2025). Written informed consent was obtained from the parents of the patient.

GAIT ANALYSIS

In this study, gait function was evaluated using multiple instrumented 3DGA datasets and the Movement Analysis Profile (MAP) quantitative scoring system, which includes Gait Variable Scores (GVS) and the Gait Profile Score (GPS). 3DGA data were acquired using the MXF-20 system (Vicon, Oxford, UK) in conjunction with two force plates (AMTI, Watertown, MA, USA). Kinematic and kinetic data were calculated using the Plug-in Gait model (Vicon). MAP enables visualization of kinematic data via simplified bar charts, providing enhanced interpretability when compared with traditional graphical analysis.

MAP consists of GVS and GPS, as originally described by Baker et al.^4^^)^ Both GVS and GPS are expressed in degrees. In GVS charts, greater bar heights indicate greater deviations from normative control values for each joint motion value, whereas larger values in GPS charts represent poorer overall gait function. GVS is calculated as the root mean square difference between an individual’s kinematic data and a control dataset, encompassing nine kinematic parameters: pelvic tilt, pelvic obliquity, pelvic rotation, hip flexion, hip abduction, hip rotation, knee flexion, ankle dorsiflexion, and foot progression. The control dataset consisted of normative data obtained from 38 typically developing children aged below 18 years, collected between 2005 and 2007 in Melbourne, Australia.^5^^)^ GPS is derived as the root mean square average of the GVS variables.

Preoperative and postoperative (15 months) GVS data are shown in Fig. 5A, B. Preoperatively, the patient exhibited elevated GVS in knee flexion/extension (right, 22.9°; left, 20.0°), ankle dorsiflexion/plantar flexion (right, 17.5°; left, 18.4°), and hip internal/external rotation (right, 41.5°; left, 26.9°). At 15 months postoperatively, 3DGA revealed significant improvements in GVS for knee flexion/extension (right, 6.3°; left, 7.7°), ankle dorsiflexion/plantar flexion (right, 8.6°; left, 10.9°), and hip internal/external rotation (right, 10.8°; left, 9.2°). Furthermore, the GPS improved by 9.5°, from 17.2° preoperatively to 7.7° (Fig. 5C). The minimal clinically important difference (MCID) for GPS is reported as 1.6°; in the current case, the improvement observed in GPS was 5.9 times the MCID.^4^^)^

Knee joint kinematics were evaluated preoperatively and postoperatively. The preoperative and postoperative sagittal plane moments of the bilateral knee joints are presented in Fig. 6. The peak knee flexion moment during the stance phase decreased from 0.92 Nmm/kg preoperatively to 0.52 Nmm/kg postoperatively on the right side, and from 0.59 Nmm/kg preoperatively to 0.46 Nmm/kg postoperatively on the left side.

The preoperative and postoperative coronal plane moments of the bilateral knee joints are shown in Fig. 7. The peak knee varus moment during the stance phase improved from 0.81 Nmm/kg preoperatively to 0.25 Nmm/kg postoperatively on the right side, and from 0.92 Nmm/kg preoperatively to 0.05 Nmm/kg postoperatively on the left side.

DISCUSSION

The primary goals of treating patients with severe varus deformity associated with achondroplasia are to improve lower limb alignment and gait patterns. Pfeiffer et al.^6^^)^ conducted interviews with children and adolescents with achondroplasia to assess impacts on physical health, function, school life, psychological well-being, and social well-being. Among the functional challenges reported, difficulty with prolonged and long-distance walking was frequently cited. The current treatment approach may improve gait and reduce challenges associated with extended walking.

Kiernan^7^^)^ conducted gait analysis in children with achondroplasia and identified excessive hip abduction during both stance and swing phases. Additionally, Sims et al.^8^^)^ compared 3DGA results between adults with achondroplasia and healthy adults, finding significantly greater GVS deviations in hip internal–external rotation, knee flexion–extension, and ankle dorsiflexion–plantarflexion among individuals with achondroplasia. Furthermore, GPS values were 64% higher than those of healthy individuals. In the present study, similar GVS deviations were observed preoperatively; however, improvements were noted postoperatively following limb lengthening surgery, resulting in a gait pattern more closely approximating typical ambulation.

Regarding moment analysis, Broström et al.^9^^)^ reported that individuals with achondroplasia exhibit anterior pelvic tilt, which increases hip and knee flexion moments and may contribute to low back, joint, and leg pain. The current study demonstrated reductions in both knee flexion and varus moments. These improvements may enhance gait efficiency and reduce pain and fatigability.

The decrease in knee flexion moment following limb lengthening surgery is considered to result from the correction of varus alignment and the normalization of the lower limb mechanical axis, as evidenced by the change in ZMA from −3 to +1. This correction likely contributed to a shift in the mechanical force vector of the knee joint toward a more physiological alignment. Furthermore, postoperative rehabilitation and gait training may have improved muscular balance around the knee, particularly in the quadriceps and hamstrings, enabling more appropriate muscle activation, which in turn may have led to the decrease in the knee flexion moment.

At our institution, 6MWT is conducted with the assistance of physical therapists. Preoperative and postoperative comparisons revealed an increase in walking distance from 367 m to 411 m and cadence from 124 steps/min to 132 steps/min. In addition, subjective fatigue improved significantly from 7.5 to 1 on a 10-point scale.

The PCI in the 6MWT is calculated as [(post-exercise heart rate – resting heart rate) / walking speed] (m/min). PCI is a reliable and valid indicator of energy expenditure in individuals with movement disorders.^10^^)^ In this case, as shown by 3DGA results, notable changes in gait patterns were observed preoperatively and postoperatively, underscoring the importance of rehabilitation for gait correction.

Barker et al.^11^^)^ reported that muscle strength may decline slightly following limb lengthening surgery but generally does not impair daily functional capacity. In this case, a temporary decline in activity performance was noted postoperatively, followed by improvement at final follow-up. Enhanced 6MWT outcomes were likely attributable not only to improvements in alignment and gait mechanics from surgery but also to the effects of rehabilitation through gait correction and physical conditioning.

CONCLUSION

This case demonstrated improvements in both static lower limb alignment and gait function. Although static alignment assessment using plain radiography is straightforward and is widely used, its evaluation capacity is limited. Integrating gait assessment tools such as the 6MWT and 3DGA with radiographic imaging enables a more comprehensive and quantitative evaluation before and after surgery, potentially enhancing treatment outcomes.