In this study, we investigated the feasibility of B-ultrasound measurement for evaluation of muscle recovery following free functioning gracilis transfer. The results showed that The M ≥ 4 group had significantly higher CR1, CR2, and ROM value (joint mobility) as compared with the M < 4 group. The CR1 > CR2 group had significantly higher CR1, muscle strength, and ROM than the CR1 ≤ CR2 group. The MBR > 1 group had significantly higher muscle strength than the MBR ≤ 1 group. CR1 value was highly correlated with muscle strength (r = 0.808) and ROM (r = 0.847), while CR2 value was moderately correlated with muscle strength (r = 0.491) and ROM (r = 0.556). Multivariate linear regression analysis showed that higher CR1/CR2 value was associated with higher muscle strength and higher joint mobility. Meanwhile, the CR1 > CR2 group had better muscle strength and ROM than the CR1 ≤ CR2 groups (all P < 0.01). Taken together, these results suggested that B-ultrasound measurement can quantitatively reflect muscle function following gracilis transfer, and CR but not MBR value could be a potential indicator for muscle function recovery.
Currently, ultrasonic parameters which could be used as the indicator for muscle contraction include echo intensity (EI), muscle thickness, muscle fiber pennation, measures of muscle architecture. EI reflects muscle function by the density of high echo signals in the muscle but is susceptible to be affected by the connective tissue between the subcutaneous and muscle bundles. A study on the changes in muscle strength by Jacobs et al. has confirmed that EI is not an optimal indicator of muscle strength. In addition, the transplanted gracilis muscle undergoes denervation and fibrosis, therefore EI is not suitable for function evaluation following free functioning gracilis transfer. Muscle fiber pennation reflects the muscle contraction by the angle between the direction of muscle fibers and the long axis of the muscle. Gracilis muscle is a non-feathery muscle, and could not be measured by muscle fiber pennation. The transplanted muscle is close to the skins, and the ultrasonic test is susceptible to be affected by probe-induced pressure, hence, muscle thickness measurement is also not applicable for muscle transplantation.
The gracilis muscle is non-feathery muscle, and its muscle fibers are arranged in the same direction as the tendon. During the muscle contraction, all the muscle fibers slide in parallel. Therefore, the histological cross-sectional area of the muscle fibers of the gracilis muscle is substantially the same as the gross anatomical cross-sectional area. Therefore, B-ultrasound can be used to measure CSA, and the dynamic contraction of muscles can be reflected by CSA changes (ie, CR) at rest and contraction. The transplanted gracilis muscle undergo denervation and nerve re-innervation, and the muscle volume changes during this process. The MBR and CR are standardized indicators calculated based on CSA, which can eliminate the impact of muscle volume change during denervation and nerve re-innervation. Hence, we chose CR and MBR as the indicators to explore the recovery of gracilis muscle functional after free functioning gracilis transfer.
Our results showed a high correlation between CR value and muscle function indexes, indicating that CR can be used to dynamically evaluate the recovery of muscle function after transplantation. In addition, muscle strength and ROM were significantly higher in the CR1 > CR2 group than in the CR1 ≤ CR2 group, suggesting that when the muscle CR increases after transplantation, the patient has better recovery of muscle strength and joint mobility. We observed an increase in the CR value of the gracilis muscle after transplantation. One of the possible reasons is as follows: Since the motion range of elbow bowing is larger than hip adduction, the muscle fibers of transplanted gracilis muscle need to be parallel sliding for a longer distance in the elbow bowing. Therefore, the CR value was elevated after transplantation. Our results showed that the mean CR1 values of the muscle strength ≥ M4 group and the CR1 > CR2 group were 1.35 ± 0.10 and 1.29 ± 0.15, respectively. Based on these results, we propose that CR1 of 1.3 might be an important reference value for transplanted muscle recovery. When the CR value of transplanted muscle reaches 1.3, the muscle might have satisfactory recovery. However, this reference value should be further validated in a large trial.
In this study, we also evaluated MBR, which reflects muscle volume change, ie atrophy (MBR < 1) or hypertrophy (MBR > 1). We found that patients who had no atrophy with gracilis muscle (MBR > 1) showed no significant advantage in muscle strength and joint mobility. Subgroup analysis of muscle strength also showed that MBR value was not significantly different between two muscle strength groups. Correlation analysis also revealed that there was no correlation between MBR and muscle strength/joint mobility. All these results suggested that MBR is not suitable as an indicator for evaluating muscle recovery following free functioning gracilis transfer.
Our preliminary findings demonstrated the feasibility of utilizing B ultrasound to evaluate functional recovery after gracilis transfer. Early muscle contraction changes may not be easily detected by physical examination. B-ultrasound examination can dynamically detect the progress of muscle contraction recovery, as well as the tendon-gliding function. When a lower postoperative CR value is found in early postoperative period, the patients can seek the help of rehabilitation doctors for physical therapy to promote nerve reinnervation. If in the late postoperative period (e.g. at 1 year after surgery, there is no progress in muscle streagth for 3 consecutive months), a second femoral muscle transplantation could be performed to rebuild the flexor. Even though this was a prospective study, however, the sample size was relatively small. However, In the future, a large trial should be conducted to validate the findings of this study and define the reference values for functional recovery assessment.