Muscle force and power following tendon repair at altered tendon length.

Background: While a great deal is known regarding the performance of muscle with intact tendon, little is known about muscle performance when tendon is surgically lengthened or shortened. This knowledge may allow surgeons to more accurately predict functional outcome following tendon repair when correcting a simple tendon laceration or performing a more complex vascularized neuromuscular transfer.

Materials And Methods: We studied muscle performance 12 wk following extensor tendon repairs producing altered tendon lengths. Forty male Fischer 344 rats underwent division of the proximal and distal tendons of the extensor digitorum longus muscle. Tendons were immediately repaired producing tendons with increased length, decreased length, or presurgical length (control). Observation confirmed that altered tendon length produced inverse changes in initial resting muscle tension.

Results: Muscle in the decreased tendon length group demonstrated a 15.2% greater muscle mass, 4.9% greater muscle length, 9.6% greater physiological cross-sectional area, 12.6% greater maximum isometric force, and 31.9% greater maximum power relative to the control tendon length group (P < 0.05). The increased tendon length group did not differ significantly from the control tendon length group for any measurement. Histologically, muscles set with a decreased tendon length demonstrated normal appearing hypertrophied fibers, without evidence of detrimental histological effects such as fibrosis, denervation, necrosis, inflammation, fiber type changes, or fiber splitting.

Conclusion: These data support the clinical practice of setting muscles with increased passive tension when performing tendon transfer surgeries. Conversely, setting muscles with decreased tension does not necessarily result in a force or power deficit.