Overcorrection of the inter-metatarsal angle in corrective hallux valgus osteotomy leads to an increase in intra-articular pressures of the 1st metatarsal-phalangeal joint: a biomechanical simulation

Introduction

Despite the multiple patho-aetiological basis of Hallux valgus (HV), corrective osteotomy is a common and globally performed orthopaedic procedure. Over-correction of the inter-metatarsal angle (IMA), however, is not without consequence. Through increased joint pressures, over-correction may predispose to joint degeneration. Hypothesizing that over-correction leads to an increase in intra-articular joint pressures, we constructed a mechanical simulation of the 1^st metatarsal-phalangeal joint.

Methods

The vector forces of Flexor Hallucis Longus, Extensor Hallucis Longus, Extensor Hallucis Brevis and Adductor Hallucis, acting upon the 1^st proximal phalanx were simulated with tensioned weighted rope. A Foley catheter balloon inflated to 1.5ml, simulated intra-articular distractive vector forces. The joint was freely mobile.

At 5° IMA intervals from 10° valgus to 15° varus, intrarticular pressures were recorded using a contact pressure (Graseby™, Hertfordshire, UK) transducer attached to a calibrated manometer (Digitech Instruments™, Ulhasnagar, India). At each angle, measurements were repeated five times with removal and recalibration of the pressure monitor after each measurement.

Results

A decline and subsequent increase in mean pressure was observed at each 5° interval from 10°valgus to neutral and then to 15° varus (mean±s.d.; 48.02±5.67, 24.72±1.01, 12.34±0.84, 18.96±2.46, 32.88±5.06, 42.92±4.99 milibar for 10°, 5° valgus, neutral, 5°, 10°and 15° varus IMA). One-way analysis of variance with post-hoc Tukey multiple comparison test revealed a significant increase in pressure from neutral and 5° valgus to 10° and 15° varus (p< 0.05).

[First Metatarsal Phalangreal joint pressures]

Conclusion

This is the first description demonstrating increases in intrarticular pressures as the IMA tended from 5° to 15° varus, replicating inatrogenic Hallux Varus iatrogenically produced through osteotomy and over-correction of the IMA. Over-correction and increased intrarticular joint pressure may have an aetiological basis to post-operative 1^st MTPJ degeneration. Over-correction is not a benign entity requiring consideration in Hallux Valgus corrective osteotomy.