“Proportionate Lengthening”: Developing a Novel Measurement of the Acutely Ruptured Achilles Tendon and a Potential Correlate of Function

Achilles tendon (AT) ruptures are a common orthopedic injury, with non-operative treatment increasingly recommended in the literature due to its lower risk profile and comparable outcomes to surgery1,2. However, tendon lengthening due to the initial tear gap and post-treatment elongation may compromise functional outcomes1,3,4. Some surgeons select operative or non-operative management using absolute tear gap size, but this may have different implications based on individual anatomy3. This study investigated the relationship between ‘proportionate lengthening’ (PL), the ratio of the rupture gap size to total tendon length, and functional outcomes to enhance prognostic accuracy and inform individualized treatment. Patients with a tendon rupture gap of Musculoskeletal radiographers and trained research staff measured tendon length and strength. The Achilles Tendon Rupture Score (ATRS) and Tegner Activity Scores were recorded at baseline, six-months, and 12-months. Correlation analyses quantified the association between PL and outcomes. Twenty-seven patients consented to participate. The sample consisted of 24 (88.9%) males with a mean age of 41.3±8.4 years. The mean rupture gap was 0.6±0.5cm and the mean uninjured tendon length was 19.4±2.5. The mean PL was calculated to be 3.0±2.7%. Mean tendon elongation from injury to 12-months follow-up (mean of 14.5±5.7 months) was 1.1±3.7cm. Isokinetic peak torque of trials one and two (r = −0.6822 and r = −0.6087) were significantly negatively correlated with PL (p = 0.0145 and p = 0.0357). The length of the healed tendon positively correlated with PL at six months post-injury while ATRS and isometric peak torque were negatively correlated with PL at six months. However, these outcomes did not reach statistical significance. Isometric peak torque (r = −0.6808) was statistically significantly correlated with PL at 12 months post-injury (p = 0.0435). No outcomes, objective or patient-reported, were statistically associated with absolute tendon rupture gap. PL negatively correlates with ATRS, isometric and isokinetic peak torque, to a greater degree than absolute rupture gap size. This preliminary evidence suggests that PL may serve as a more reliable indicator of functional outcome following AT rupture. Future studies involving functional outcomes in acute AT rupture should consider analyzing outcomes based on PL rather than the absolute tear gap.