Biomechanics of medial hamstring lengthening.

BACKGROUND: Medial hamstring lengthening is frequently used to correct contractures in neuromuscular conditions such as cerebral palsy. Surgical techniques vary considerably and little is known about the lengthening characteristics of muscle-tendon-units after surgical intervention. METHODS: A randomized trial was performed on paired cadaver medial hamstring muscle-tendon-units comparing 'High' (proximal) versus 'Low' (distal) lengthening procedures. The paired muscle-tendon-units were subjected to tensile testing utilizing an Instron® (Instron Corporation, Canton, MA, USA) machine under controlled conditions. Prior to tensile testing, the paired semitendinosus and paired gracilis received either high or low intramuscular tenotomy. Load (N) versus displacement (mm) was recorded continuously for each test. The difference in lengthening and load at failure for intact and surgical simulation muscle-tendon-units was recorded and compared with paired t-tests. RESULTS: Both low and high lengthenings increased the amount of lengthening achieved compared with intact controls and the lengthening was achieved at lower applied load. Low intramuscular tenotomy of the semitendinosus resulted in a 30% greater lengthening when compared with high intramuscular tenotomy. For the gracilis muscle, the low intramuscular tenotomy achieved 39% greater lengthening than the high intramuscular tenotomy and these differences were significant. CONCLUSION: Biomechanical testing of formalin-preserved human cadaveric medial hamstring muscle-tendon-units confirms that it is possible to achieve lengthening in continuity following an intramuscular tenotomy or fascial striping procedure. The site of the surgical procedure (high versus low) results in a different effect on the lengthening characteristics, dictated by the anatomy of the particular muscle-tendon-unit.

The effects of intramuscular tenotomy on the lengthening characteristics of tibialis posterior: high versus low intramuscular tenotomy.

BACKGROUND: Lengthening of soft-tissue contractures is frequently required in children with a wide variety of congenital and acquired deformities. However, little is known about the biomechanics of surgical procedures which are commonly used in contracture surgery, or if variations in technique may have a bearing on surgical outcomes. We investigated the hypothesis that the site of intramuscular tenotomy (IMT) within the muscle-tendon-unit (MTU) of the tibialis posterior (TP) would affect the lengthening characteristics. METHODS: We performed a randomized trial on paired cadaver tibialis posterior muscle-tendon-units (TP-MTUs). By random allocation, one of each pair of formalin-preserved TP-MTUs received a high IMT, and the other a low IMT. These were individually tensile-tested with an Instron(®) machine under controlled conditions. A graph of load (Newtons) versus displacement (millimetres) was generated for each pair of tests. The differences in lengthening and load at failure for each pair of TP-MTUs were noted and compared using paired t tests. RESULTS: We found 48% greater lengthening for low IMT compared to high IMT for a given load (P = 0.004, two tailed t test). Load at failure was also significantly lower for the low IMT. These findings confirm our hypothesis that the site of the tenotomy affects the amount of lengthening achieved. This may contribute to the reported variability in clinical outcome. CONCLUSIONS: Understanding the relationship between tenotomy site and lengthening may allow surgeons to vary the site of the tenotomy in order to achieve pre-determined surgical goals. It may be possible to control the surgical "dose" by altering the position of the intramuscular tenotomy within the muscle-tendon-unit.