Volume 145, December 2022, 111393Author links open overlay panelAbstract

Plantar flexor functional deficits measured using joint dynamometry are associated with poor outcomes in patients following Achilles tendon rupture. In this study, we developed a small animal dynamometer to quantify functional deficits in a rat Achilles tendon rupture model. Like our reported plantar flexor deficits in patients recovering from Achilles tendon ruptures, we found in our small animal model functional deficits across the ankle range of motion, resulting in an average 34% less positive work being done compared to the uninjured contralateral limb. These functional deficits are similar to 38% less plantar flexor work done by patients who were treated non-surgically in our prior research. Further, these torque deficits were greater in plantar flexion than dorsiflexion, which agree with clinical complaints of limited function during tasks like jumping and hiking. These findings serve as compelling evidence that our Sprague Dawley rat model of an Achilles tendon rupture recapitulates the functional deficits we observed in patients treated nonsurgically. We provide thorough documentation for other groups to build their own dynamometers, which can be modified to meet unique experimental criteria.

Introduction

Quantifying functional deficits following musculoskeletal injury are a cornerstone of clinically impactful research. Joint dynamometry has long been considered the ‘gold-standard’ for quantifying joint-level function in patient populations (Dvir and Müller, 2020, Martin et al., 2006, Ushiyama et al., 2017). Despite these established techniques in patient populations, there is less consensus on how to quantify functional deficits in pre-clinical models that aim to recapitulate human disease conditions. In this short communication, we describe a modifiable, low-cost, and convenient small animal dynamometer that quantifies joint-level functional capacity. We then use an established rat Achilles tendon rupture model (Freedman et al., 2016) to test our hypothesis that functional deficits are similar between our rat model and our observations in patient populations (Baxter et al., 2018, Hullfish et al., 2019).

Section snippetsStudy overview and dynamometer description

We designed this small animal dynamometer to be modifiable to accommodate a range of animal species and joints tested. The dynamometer has 8 primary components (Fig. 1): 1) a platform constructed from 2 aluminum breadboards with a grid of threaded holes to secure instrumentation and 3D printed parts, 2) an instrumented foot plate assembly comprised of a torque cell in series with a servomotor and 3D printed foot plate, 3) 3D printed parts to safely secure the test animal to the dynamometer, 4)

Results

The Achilles tendon rupture caused plantar flexor functional deficits across the ankle range of motion (Fig. 2). When normalized to the contralateral uninjured ankle, we found that these functional deficits were most pronounced in plantar flexion. Peak plantar flexor torques were 30–34 % less between 60 degrees dorsiflexion to 20 degrees plantar flexion (p < 0.001). These deficits increased to 40 % in 40 degrees plantar flexion and to 48 % in 60 degrees plantar flexion. These functional

Discussion

We developed this small animal dynamometer to quantify plantar flexor functional capacity in a rat Achilles tendon injury model. We selected geometries, motor size, and torque cell range that met these experimental criteria. However, each of these components can easily be replaced or modified to meet the demands of other small animals or joints. For example, testing a larger animal like a rabbit could be done by using a torque cell with greater measurement range and a servomotor capable of

Funding

This work was supported by NIH/NIAMS K01AR075877 and P30AR069619.

Preprint

A version of this manuscript is publicly posted on bioRxiv at https://doi.org/10.1101/2022.08.01.502343.

Signifigance

Preclinical models are critical tools for translating knowledge discovery to clinical decision making. We developed a low-cost and modifiable joint dynamometer that quantifies joint function in small animals. Here, we used a rat model to test the implications of Achilles tendon ruptures that were not repaired on plantar flexor function. We found that Achilles tendon ruptures in a rodent model closely resemble the functional deficits our group has observed in patients.

CRediT authorship contribution statement

My M. Tang: Conceptualization, Methodology, Investigation, Writing – review & editing, Writing – original draft. Courtney A. Nuss: Writing – review & editing, Methodology. Natalie Fogarty: Writing – review & editing, Methodology. Josh R. Baxter: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank Mr. Todd Hullfish, Mr. Hammo Ahmad, and Ms. Liala Sofi for assisting in the small animal surgeries and immobilization protocol.

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