THE EFFECT OF KNEE AND ANKLE POSITION ON DISPLACEMENT OF ACHILLES TENDON RUPTURES IN A CADAVERIC MODEL: IMPLICATIONS FOR NON-OPERATIVE MANAGEMENT

Participants: J. K. Sekiya, K.E. Evensen, P. J. L. Jebson, J. E. Kuhn

Keywords: Achilles tendon ruptures, conservative treatment, cadaver/anatomic study

Introduction

The treatment of a ruptured Achilles tendon is controversial. There are proponents of surgical repair, as well as non-operative treatment. Within each approach are a variety of methods to achieve the end result. With non-operative treatment, the method and position of immobilization recommended has been extremely variable. While many authors have advocated short leg immobilization in a degree of ankle plantar flexion established by gravity, others have recommended forced ankle equinus, and yet others recommend long leg casting to prevent knee and gastrocnemius movement. There is clearly no consensus on the appropriate method of immobilization for the non-operative treatment of an Achilles tendon rupture.

The purpose of this experiment was to analyze the effect of knee flexion angle, ankle plantar flexion angle, and location of the defect on the separation of the severed ends of the Achilles tendon, in a reproducible cadaver model. With this data we hope to provide insight into the appropriate method and position of lower limb immobilization for non-operative treatment of an Achilles tendon rupture.

Materials and Methods

Six fresh frozen human (average age 90.7 years, 1 male and 2 female) cadaver legs, harvested from mid thigh to toes, were used. Specimens were thawed and a 6 cm posterior approach to the Achilles tendon was made through the skin and peritenon. A transverse incision was made perpendicular to the orientation of the Achilles tendon fibers in each specimen. In four specimens, the transverse incision in the Achilles tendon was made 4 cm from the insertion on the calcaneus. In one specimen, the incision was made 6 cm from the calcaneus, and in one specimen the incision was made 2 cm from the calcaneus. In each specimen, 4-0 stainless steel wire suture was secured into each severed tendon end 5 mm from, and parallel to, the transverse incision. The skin and peritenon were closed with non-absorbable suture.

An external fixator hinged at the knee and a foot plate hinged at the ankle was applied to the anterior femur and tibia of each specimen. Radiographs were taken at the site of the Achilles tendon incision during testing. Testing positions for each specimen included 0° , 20° , 40° , and 60° of ankle plantar flexion. For each of these positions radiographs were obtained with the knee flexed to 0° , 40° , 80° , and 120° . Radiographs were used to measure the distance in millimeters between the wire suture markers on each end of the severed tendon.

Statistical analysis was conducted using a two by two repeated measures ANOVA test, with a Bonferonni Post-Hoc test to correct for multiple comparisons. The two treatment groups were knee angle and ankle angle, with the dependent variable of gap distance, measured in millimeters. Statistical significance was set at p<0.05.

Results

As expected, the degree of ankle plantar flexion had a significant influence on the displacement of the Achilles tendon edges (p<0.001). As the ankle is progressively plantar flexed, the tendon edges re-approximate. At 60° of plantar flexion, the tendon edges, on average, were fully opposed and in some cases overlapped. The magnitude of the change was clinically significant, as in the 0° position, the tendon edges averaged over 20 mm of displacement. At 60° of ankle plantar flexion, the position of the knee had little influence on the amount of tendon displacement (p=0.43). In fact, although a trend is noted such that knee extension created some displacement of the tendon edges, this was not a significant finding (p=0.055), and the magnitude of displacement averaged only 3 mm. Interestingly, we detected no association (interaction) between knee flexion angle and ankle plantar flexion angle in this study (p=0.994).

Discussion

Our study suggests that ankle position is a critical component in re-approximating the Achilles tendon ends to facilitate healing in an anatomic position. We found 60° of ankle plantar flexion is necessary to satisfactorily re-approximate the severed ends of the Achilles tendon. The normal range of ankle plantar flexion varies anywhere from 15° to 50° depending on age, sex, and the landmarks used to measure ankle equinus. This finding implies that ankle immobilization in forced equinus is preferable to gravity equinus.

In addition, although a relationship between tendon gapping and knee extension was noted, this finding was not statistically significant, nor clinically significant as the total average gap created by extending the knee from 120° of flexion to full extension was only 3 mm. With the ankle in 60° of plantar flexion, the position of knee immobilization had no appreciable influence on the tendon gap. These findings suggest that including the knee during immobilization is not necessary in the non-operative treatment of an acute Achilles tendon rupture.

There are limitations to our cadaveric model. We could not reproduce the actual injury and sequellae of the injury. As a result, in-vivo, non-recoverable tendon strain may occur, lengthening the tendon prior to failure. In addition, hemorrhage in the tendon sheath may further limit tendon opposition. Finally, in the patient with an acute Achilles tendon rupture, active gastrocnemius muscle activity may occur, which could potentially cause the tendon gap to increase. These issues could not be addressed in our cadaver model. Nevertheless, our study suggests that immobilization of the ankle in maximal plantar flexion is the critical component to achieve anatomic positioning of the torn Achilles tendon, and that knee immobilization is not necessary.