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.