Discussion
This present study showed that the shear moduli of the VM and VL on the affected side in patients after knee joint arthroscopic surgery were significantly lower than the unaffected side in the contraction condition, but no significant differences in the resting condition. The muscle thickness and pennation angle of the three muscles were not significantly different in the affected and unaffected sides in the patient group. Additionally, the contraction ratio of the VM, VL and RF in the control, unaffected side and affected side had no significant muscle-by-limb interaction.
Muscle force cannot be quantified directly from the shear modulus of the contraction condition of the muscle. However, it can be used to estimate the change of muscle force from change of shear modulus, because muscle force and shear modulus have a strong linear relationship. Sasaki et al investigated the relationship between ankle joint torque and the shear modulus of the tibialis anterior muscle.7 The results indicated a linear association between muscle force and the shear modulus (R2=0.52). In the control group, the shear modulus of each muscle showed no interaction, so the change from resting condition to contraction condition showed the same pattern on the left and right side. On the other hand, the shear moduli of the VM and VL in the patient group showed different patterns between the unaffected and affected side. It can be assumed that there was no pathological change in the mechanical properties of the muscle after surgery in the patient group, since there were no significant difference in shear modulus between the unaffected side and the affected side at resting condition. Therefore, on the affected side of the patients, it is suggested that muscle force may be decreased corresponding to a decrease in shear modulus.
It is reported that knee joint extension torque decreased by 61% at 1 month after knee arthroscopic surgery. Furthermore, several studies using surface electromyogram (EMG) showed that the muscle activity of the quadriceps femoris muscle declines in the early phase after knee joint surgery. The muscle activity during isometric knee joint extension movement decreased by 37% in 2 weeks after surgery.3 In addition, when isometric leg press was performed at 2 weeks after surgery, it decreased by 31%.2
These results from previous studies clearly demonstrate the low function of the quadriceps femoris muscle of the knee joint postsurgery. In the present study, the shear moduli of the VM and VL on the affected side were 30% (68.3 vs 20.8 kPs) and 52.3% (63.3 vs 33.1 kPa), respectively, on the unaffected side at maximum isometric contraction of the quadriceps femoris muscle with knee joint extension. These results are similar to the reported muscle activity and knee extension torque.
It was suggested that the change in muscle architecture does not influence changes in the shear modulus of the VM, VL and RF because there were no differences between the unaffected and affected sides with regard to muscle thickness and pennation angle, whether the muscle was contracted or not. It was reported that muscle architecture is possibly an index of the force-generating capacity of the muscle.10 12 However, it was difficult to predict a significant decrease of the shear modulus on the affected side in the present study from the change in muscle architecture with muscle contraction. Thus, it was expected that the measurement of the shear modulus is a clinical indicator that can sensitively evaluate decreased contraction of muscle.
There were no significant differences in the shear modulus of the RF between the unaffected side and the affected side in the contraction condition (figure 4F). The contraction ratios of the VM and VL were significantly larger than those of the RF when collapsed across measurement limbs (figure 5B). This may be because of the influence of measurement position. When the hip joint is in flexion position, the RF, which is a biarticular muscle, is relaxed and the contraction force decreases. Therefore, the difference in the shear modulus between the unaffected side and affected side was small.
Hug et al stated that the measurement of shear modulus is an effective indicator that can estimate the muscle force of individual muscles in the agonist muscles.13 However, the contraction ratio of the VM, VL and RF in the control, unaffected side and affected side had no significant muscle-by-limb interaction (figure 5A). We considered that the balance of the contraction ratio between muscles on the affected side had not collapsed due to surgery.
Some factors have been reported to cause muscle dysfunction postsurgery. Arthrogenic muscle inhibition (AMI), which is a mechanism of reflex inhibition in the nervous system, may be strongly related to intraarticular swelling postsurgery.14 Given that atrophy of the quadriceps femoris muscle is related to a decrease of the force-generating capacity of the muscle,15 factors such as AMI and muscle atrophy may be the reason for the difference between the unaffected and affected sides of the shear modulus; however, it was difficult to identify these factors from the data in the present study. Further comprehensive examinations related to establishing an index of muscle architecture, muscle activation and force-generating capacity of the muscle will be necessary in the future.
The evaluation of muscle function using shear-wave elastography, as demonstrated in the present study, is expected to became a useful index for measurement of muscle function.4 6 7 As mentioned above, muscle dysfunction in the early postsurgery period was assessed using surface EMG and knee extension strength measuring equipment. However, the value measured by surface EMG possibly has low validity in a comparison of daily variation and difference between the unaffected and affected side. In addition, there are patients who have difficulty performing knee extension at maximal effort because of pain during the early postsurgical period. Drechsler et al reported that 71% (22/31) of patients could not accomplish the knee extension task 1 month after surgery.1 It was difficult to evaluate dysfunction of the quadriceps femoris muscle with accuracy during the early postsurgery period due to physical burden and non-quantitative evaluation. This study showed that shear-wave elastography successfully and quantitatively evaluated the shear modulus without being influenced by the position of the patients. Therefore, we suggest that the measurement of the shear modulus has potential as a new evaluation index with high clinical value as it is quite sensitive to decreases in muscle contraction.
Limitations
This study has a few limitations. First, we could not measure knee extension torque. The reason was we wanted to prevent aggravation of inflammation and hyperstress of the repair tissue. It was reported that the shear modulus is related to contraction intensity of the biceps brachii muscle and the tibialis anterior muscle, but there was no study on the quadriceps femoris muscle. Therefore, it will be necessary to investigate the relationship between the shear modulus and contraction intensity in the quadriceps femoris muscle in the future. Second, we did not assess the shear modulus, presurgery and during follow-up. The previous study reported that the EMG amplitude of the VM lowered 9 hours postsurgery compared with the presurgical value.2 However, functional deterioration before surgery may have possibly existed. It will be necessary to assess the presurgical shear modulus of the quadriceps femoris muscle. In addition, it will be important to investigate the postoperative progress in the future. The follow-up study estimates the recovery of the quadriceps femoris muscle function and may become an index of progress and may be used to measure the effects of rehabilitation. Third, the vastus intermedius (VI) is also a muscle of the quadriceps, but we evaluated only the VM, VL and RF. The reason is that the shear wave did not reach the muscle belly of the VI that was deeply located. Therefore, it will be necessary to clarify the active properties of the quadriceps femoris muscle, including the VI, in the future.