Evaluation of the effects of acute lower extremity lengthening in total hip arthroplasty on femoral and sciatic nerves with intraoperative neuromonitorization

Aim: Nerve injury is a rare yet potentially devastating complication in total hip arthroplasty. One of the most important risk factors for nerve injury is excessive lengthening of the lower extremity. Lower extremity lengthening during total hip arthroplasty occurs especially in cases of coxarthrosis secondary to developmental hip dysplasia. Although there is a lot of literature on the amount of lengthening, the correlation between the amount of lengthening and patient's height is not clearly defined yet. Our aim in this study is to assess the effects of lengthening in total hip arthroplasty on neural structures and evaluate the relationship between the amount of lengthening and patient's total height and femoral length. Patients and Methods: Institutional Review Board of Istanbul Medical Faculty of Istanbul University approval was obtained (22.12.2017/21-1540). A priori power analysis showed 16 patients were required to obtain a power of 0,6 with a margin error of 0,05. Patients with history of spinal surgery, diabetes mellitus, neurological disease were excluded. Height, weight, distance between anterior superior iliac spine and medial malleolus (ASIS-MM) and total femur length were measured preoperatively. To perform intraoperative neuro-monitorization (IONM), patients were operated under total intravenous anesthesia. Somatosensorial evoked potentials (SEP) from posterior tibial nerve, motor evoked potentials (MEP) for deep peroneal nerve, gastrocnemius MEP for tibial nerve and quadriceps MEP for femoral nerve were recorded. During the procedure, for purposes of scaling, a 3 cm sterile ball was placed in the acetabulum after femoral neck cut and lower extremity traction was performed. The thresholds for disruption of neural conduction were set at 50% or more decrease in SEP and MEP amplitudes and 10% or more lengthening in the SEP latency. Fluoroscopic views of hip were obtained first when any of the measured potentials was affected and then when one of the thresholds were met. The absolute amount of lengthening was calculated from the fluoroscopic view using the scaling marker. Findings: 13 female and 3 male patients were in the study for a total of 16. Mean age of the patients was 51,7±11,4. Mean height was 162,2±7,2 cm. Mean weight was 72,4±17,8 kg. Mean BMI was 27,57±6,67 kg/m2. Mean ASIS-MM distance was 840,7±43,24 mm. Mean total femoral length was 454±25,8. For all patients, first change during the IONM occurred in the deep peroneal nerve. Similarly, threshold values to stop traction were achieved in the deep peroneal nerve with a decrease of 50% in the MEP amplitude in all cases. Mean amount of lengthening at the time when the IONM values were first affected was 14,9±6,1 mm; and the ratio between this value and femoral length was on average %3. Mean amount of lengthening at the time when a decrease of 50% was reached was 22,5±5,6 mm; and the ratio between this value and femoral length was %5. Mean decrease in tibial MEP amplitudes was 27,5±9,9%; and in femoral MEP amplitudes 12,6±10,5%. Tibial SEP amplitudes were unchanged in all cases. There was positive correlation between height, ASIS-MM distance, femoral length and total amount of lengthening. (r:0,835; p=0,001, r:0,811; p<0,001 ve r: 0,782; p<0,001). There was no statistically significant correlation between amount of lengthening and decrease in amplitude in tibial and femoral nerves. (p>0,05). There was no statistically significant correlation between body mass index and the total amount of lengthening (p: 0.778). Conclusion: The first nerve to be affected from lower extremity traction is the deep peroneal nerve. Mean amount of lengthening at which threshold levels for deep peroneal nerve are reached is directly correlated with total height and femoral length. There is no statistically significant correlation between amount of lengthening and decrease in amplitudes of tibial and femoral nerves when the deep peroneal nerve values reach the threshold. There is no statistically significant correlation between body mass index and nerve injury.