Psoas weakness following oblique lateral interbody fusion surgery: a prospective observational study with an isokinetic dynamometer.

Background Context: Although the surgical corridor used for oblique lateral interbody fusion (OLIF) protects the intrapsoas nerves by causing minimal compression, transient weakness remains the most commonly reported postoperative complication.

Purpose: Using a dynamometer to evaluate how the hip flexor strength changes following OLIF.

Study Design/setting: A prospective observational study.

Patient Sample: Forty-six patients who underwent single or multi-level OLIF for lumbar spondylolisthesis.

Outcome Measures: Isokinetic dynamometer values (peak torque, total work, average power), visual analogue scale (VAS) scores for leg pain, hypoesthesia, subjective weakness of the left hip flexor muscle, Oswestry disability index, body mass index, bone mineral density, radiologic findings of the psoas muscle (cross-sectional area, Hounsfield unit (HU), fat portion grade), and psoas retraction time.

Methods: The isokinetic muscle strength of the hip flexor was measured five times (preoperatively and postoperatively at 2 days, 1 week, 1 month, and 3 months) for both legs. The peak torque was defined as the postoperative strength of the left hip flexor muscles, and was compared to the preoperative baseline value. The strength of the left and right hip flexor muscles were also compared at each time point. For logistic regression analysis, when the peak torque was below the median value, it was defined as lower peak torque.

Results: Up to 1 week after surgery, the strength of the left hip flexor muscle decreased significantly (paired difference in peak torque was 22.6%, p<.001). In the results of multivariate logistic regression analysis, diabetes (odds ratio [OR]=8.43, p=.020) and the HU of the psoas muscle (OR=0.916, p=.034) were associated with lower peak torque 1 week after surgery. From 1 month after surgery, postoperative weakness of the psoas muscle was not significant. In the questionnaire survey, subjective left hip flexion weakness was reported in 8.5% (4/47) of patients 1 week after surgery, and it remained in only 2.1% (1/47) of patients after 3 months of operation. The frequency of left anterior thigh pain and hypoesthesia decreased from 85.1% (40/47) at 1 week to 2.1% (1/47) at 3 months after surgery. The mean VAS score for left anterior thigh or groin pain decreased significantly at 1 month after surgery (PO2D: 4.04±1.84, PO1M: 1.67±1.10, p<.001).

Conclusions: Dynamometer measurement showed that psoas strength declined significantly up to 1 week after OLIF surgery. Patients with diabetes or lower HU of the psoas muscle showed delayed recovery from postoperative weakness of the psoas muscle. However, the weakness was insignificant from 1 month after surgery. At 3 months after surgery, the other psoas-related problems (left anterior thigh pain and hypoesthesia) also disappeared.