Effect of too-long anterior process on plantar pressure, gait biomechanics, and clinical outcomes in athletes with chronic lateral ankle instability: a cross-sectional comparative study

The too-long anterior process (TLAP) of the calcaneus is a congenital elongation often overlooked in patients with chronic lateral ankle instability (CLAI), which may contribute to persistent sinus tarsi pain and refractory symptoms. The impact of TLAP on plantar pressure distribution, gait biomechanics, and clinical outcomes in athletes with CLAI remains unclear. Fifty athletes (50 feet) with unilateral CLAI were divided into the TLAP group (calcaneonavicular distance [CND] < 5.0 mm on computed tomography, n = 25) and the control group (CND ≥ 5.0 mm, n = 25). Plantar pressure and spatiotemporal gait parameters were measured using the Zebris FDM-T system. Surface electromyography (sEMG) of peroneus longus (PL), tibialis anterior (TA), and medial gastrocnemius (MG) were recorded simultaneously during a gait cycle. Clinical outcomes included the American Orthopaedic Foot and Ankle Society (AOFAS) ankle-hindfoot scale, visual analogue scale (VAS), and Foot and Ankle Outcome Score (FAOS). The two groups had comparable baseline characteristics. Compared with the control group, the TLAP group showed significantly higher peak pressure in the lateral hindfoot (135.6 ± 21.4 vs. 107.8 ± 18.2 kPa, P < 0.001) and lateral midfoot (29.5 ± 7.6 vs. 20.7 ± 6.1 kPa, P < 0.001), with a more lateralized center of pressure ( P < 0.001). Gait analysis revealed a prolonged stance phase (67.8 ± 2.5% vs. 64.3 ± 2.1%, P < 0.001), increased double support (34.2 ± 2.5% vs. 30.3 ± 1.9%, P < 0.001), and reduced gait velocity (2.4 ± 0.4 vs. 2.8 ± 0.3 km/h, P < 0.001) in the TLAP group. sEMG showed lower PL activity during the loading response (18.3 ± 5.1% MVIC vs. 25.6 ± 6.4%, P < 0.001). Clinically, TLAP patients had lower AOFAS scores (68.2 ± 7.4 vs. 73.5 ± 6.0, P = 0.008), higher VAS scores (4.4 ± 1.4 vs. 3.6 ± 1.2, P = 0.041), and worse FAOS-symptom (62.6 ± 9.0 vs. 70.2 ± 8.6, P = 0.004) and FAOS-Sport (61.8 ± 14.1 vs. 69.2 ± 10.8, P = 0.043) scores. After FDR correction, AOFAS and FAOS-Symptom remained statistically significant. CND was positively correlated with AOFAS scores ( r = 0.42, P = 0.036) and PL RMS amplitude during loading ( r = 0.45, P = 0.024), and negatively correlated with lateral midfoot pressure ( r = − 0.50, P = 0.012). In athletes with CLAI, concurrent TLAP is associated with lateralized plantar overload, altered gait timing, impaired peroneal activation, and exploratorily poorer functional outcomes. These findings support the consideration of CT evaluation for TLAP in athletes with refractory CLAI who fail conservative treatment.