Achilles Tendon Ruptures in Middle-Aged Rats Heal Poorly Compared With Those in Young and Old Rats

Abstract

Background: Achilles tendon ruptures are painful and debilitating injuries and are most common in middle-aged patients. There is a lack of understanding of the underlying causes for increased rupture rates in middle-aged patients and how healing outcomes after a rupture might be affected by patient age. Therefore, the objective of this study was to define age-specific Achilles tendon healing by assessing ankle functional outcomes and Achilles tendon mechanical and histological properties after a rupture using a rat model.

Hypothesis: Rats representing the middle-aged patient population would demonstrate reduced healing capability after an Achilles tendon rupture, as demonstrated by a slower return to baseline ankle functional properties and inferior biomechanical and histological tendon properties.

Study design: Controlled laboratory study.

Methods: Fischer 344 rats were categorized by age to represent young, middle-aged, and old patients, and Achilles tendon ruptures were induced in the right hindlimb. Animals were allowed to heal and were euthanized at 3 or 6 weeks after the injury. In vivo functional assays and ultrasound imaging were performed throughout the healing period, and ex vivo tendon mechanical and histological properties were assessed after euthanasia.

Results: Rats representing middle-aged patients displayed reduced healing potential compared with the other age groups, as they demonstrated decreased recovery of in vivo functional and ultrasound assessment parameters and inferior mechanical and histological properties after an Achilles tendon rupture.

Conclusion: These findings may help explain the increased rupture rate observed clinically in middle-aged patients by suggesting that there may be altered tendon responses to daily trauma.

Clinical relevance: The results provide novel data on age-specific healing outcomes after an Achilles tendon rupture, which underscores the importance of considering a patient's age during treatment and expectations for outcomes.

Keywords: Achilles tendon; ankle; biology of tendon; biomechanics of tendon.

Figure 1.. Study Design.

Rats were aged to 5.5 months, 13 months, 19 months, referred to as “Young,” “Middle-Age,” and “Old.” Animals received Achilles tendon transection surgery, were immobilized for 1 week and were euthanized at 3 or 6 weeks post-injury. In vivo assessments and ex vivo assays performed in this study are listed including the timepoints at which they are performed (“Pre” refers to prior to injury; numbers refer to the number of weeks after injury that the assays were performed; “Un” indicates that the assay includes uninjured controls animals). SHG, Second harmonic generation imaging.

Figure 2.. Gait Kinetics and Kinematics.

(A) Normal force, (B) propulsion force, (c) braking force, and (d) medial-lateral force were all affected with injury, with the greatest effects of injury observed in the young and middle-age groups. (E) Stride width was not affected by injury but was increased with increasing age. (F) Toe spread of the injured limb was decreased with injury but recovered with time of healing in all groups. Data is displayed as mean ± SD with individual data points plotted. Bars indicate significant difference between healing timepoints. “Y” and “M” indicate significant difference relative to the young and middle-age groups at that timepoint, respectively.

Figure 3.. Passive Joint Function.

(A) Range of motion was decreased with injury in all groups, although the greatest effects of injury were observed in the middle-age and old groups. (B) Dorsiflexion and (C) plantarflexion stiffness were generally less affected by injury, although dorsiflexion stiffness does appear to decrease with age. Data is displayed as mean ± SD with individual data points plotted. Bars indicate significant difference between healing timepoints. “Y” indicates significant difference relative to the young group at that timepoint.

Figure 4.. High Frequency Ultrasound Analysis.

The FFT-based parameters (a) maximum frequency and (b) axis ratio were both affected by injury in all groups. The middle-age group is the only group that does not demonstrate improvements in maximum frequency with time of healing. Data is displayed as mean ± SD with individual data points plotted. Bars indicate significant difference between healing timepoints.

Figure 5.. Contrast Enhanced Ultrasound.

(A) Contrast intensity curves at 1 week post-injury demonstrate increased perfusion of the tendon in the young and middle-age groups relative to the old group. Line indicates mean intensity and fading indicates standard deviation of the intensity. (B) Schematic demonstrating how the intensity curves are used to generate quantitative parameters. (C) Peak enhancement, (D) rise time, (E) wash-in rate, and (F) wash-in perfusion index all indicate that the vascular response to injury is highest in the young and middle-age tendons. This vascular response is largely dissipated by 6 weeks post-injury. Data is displayed as mean ± SD with individual data points plotted. Bars indicate significant difference between healing timepoints. “Y” and “M” indicate significant difference relative to the young and middle-age groups at that timepoint, respectively.

Figure 6.. Uninjured Mechanical Assessment.

Aging results in larger tendons with increased (A) CSA. However, middle-age tendons did not show increased (B) stiffness and had reduced (C) modulus. Data was analyzed with a one-way ANOVA with Tukey’s post hoc tests. Data is displayed as mean ± SD with individual data points plotted. Bars indicate significant difference between groups.

Figure 7.. Mechanical Assessment.

(A) CSA measurements indicated transient increases at 3 weeks post-injury in middle-age and old tendons. (B) Modulus and (C) max stress values improve with healing timepoint in all groups, although modulus maintains the pre-existing decrease observed in uninjured tendons and max stress is decreased in middle-age tendons relative to young tendons. (D) Stress relaxation data indicates transient increases in viscoelasticity at 3 weeks post-injury in the middle-age and old tendons. (E) DIC analysis revealed that only the middle-age tendons had decreased regional strain heterogeneities with healing time. In addition, the (F) injured to uninjured modulus ratio was not significantly improved with healing timepoint in the middle-age group. Data is displayed as mean ± SD with individual data points plotted. Bars indicate significant difference between groups.

Figure 8.. Histological and μCT Assessment.

There were no differences in (A) cellularity within the injured region between age groups or healing timepoints. There were no differences in (B) heterotopic ossification volume between ages or healing timepoints, however (C) bone mineral density was increased in middle-age and old tendons relative to young tendons at 6 weeks post-injury. Data is displayed as mean ± SD with individual data points plotted. Bars indicate significant difference between groups.