Up to 10% of patients following Total Hip Replacement (THR) are symptomatic for a Leg Length Inequality (LLI), commonly being up to 20mm longer on the operated side. With 100,000 patients undergoing THR in 2015, 8.7% of all errors in the NHS being attributed to an LLI and malpractice claims being frequent, understanding why certain patients are symptomatic whilst others remain asymptomatic is of great importance.
Anthropometric and demographic measurements together with gait analysis results were compared between a group of 26 symptomatic LLI patients following THR, 14 asymptomatic THR patients and 38 healthy individuals using Plug-in-Gait. Statistically significant results were found for height, with LLI patients generally being 6% shorter than their THR counterparts. Gait analysis results using Visual3D and AnyBody found LLI patients demonstrated reductions in peak joint forces, ground reaction forces, moments and knee flexion relative to the THR and healthy group. This was linked to LLI patients walking 20% and 59% slower than their THR and healthy counterparts respectively. Wear analysis found that LLI patients had 9% greater sliding distances than THR patients per stride together with more unidirectional motion paths.
A thorough critique of Plug-in-Gait found the clinical results were generally reliable. Further sensitivity analyses however highlighted the weaknesses of the model if used improperly, with a 45mm error in lateral thigh marker positioning leading to a 10% change in hip flexion angle. The choice of hip joint centre regression equation, errors in joint width measurement and the use of CAST over PiG were also found to have a profound effect on kinematic results. It was concluded that LLI patients were symptomatic due to a combination of a greater LLI magnitude to height ratio, leading to greater pelvic obliquity in smaller individuals, and weakened muscles/soft tissues at the hip causing an asymmetric gait.
