Subsynovial connective tissue development in the rabbit carpal tunnel

Establishing the electrophysiological feasibility of the rabbit median nerve as an experimental model for carpal tunnel syndrome

BACKROUND

Rabbits are appropriate models for experimental carpal tunnel syndrome (CTS) studies. This study aimed to explore whether the distribution and innervation area of the nerves supplying the thenar muscles of rabbits are similar to those in humans using anatomical, electrophysiological, and histopathological methods.

NEW METHOD

20 New Zealand rabbits were used to establish reference conduction values for the median and ulnar nerves. Median nerve denervation was performed on the left forelimb of six rabbits, and changes were assessed 33 days post-surgery. Normative data from healthy rabbits were compared with those from denervated rabbits, and comparisons were made between the right and left forelimbs of the denervated rabbits. Thenar and interosseous (2nd, 3rd, and 4th) muscles were used for histopathology. Dissections focused on the thenar muscles and branches of the median and ulnar nerves.

RESULTS

Thenar muscles were 2.5-3 mm in width and 7-8 mm in length), innervated by both the median and ulnar nerves. Normative latency and amplitude values of 1.85 ± 0.30 ms and 7.71 ± 3.50 mV for the median nerve and 1.85 ± 0.19 ms and 6.14 ± 2.50 mV for the ulnar nerve, respectively. Denervation reduced thenar muscle CMAP amplitude (2.51 ± 2.16 mV, p = 0.047) on ulnar nerve stimulation, indicating dual innervation. The median sensory nerve conduction latency (2.55 ± 0.20 ms) was successfully performed for the first time. Histopathological analysis revealed localized atrophy and degenerative changes in the denervated thenar muscles.

COMPARISON WITH EXISTING METHODS

The new method including novel normative data will significantly enhance the evaluation of CTS in experimental rabbit models, paving the way for more accurate and reliable future research.

CONCLUSION

The innervation patterns of rabbit thenar muscles were similar to those in humans. This data can aid CTS understanding in rabbit models.

In vitro fatigue of human flexor digitorum tendons

Carpal tunnel syndrome and stenosing tenosynovitis (i.e., trigger finger) are common work-related musculoskeletal disorders (WMSDs) that have been linked to overuse of the flexor digitorum profundus (FDP) and flexor digitorum superficialis (FDS) tendons of the hand. These injuries occur in response to repetitive loading; as such, they may be characterized using fatigue failure phenomenon. Current WMSD evaluation tools for carpal tunnel syndrome and trigger finger are built upon fatigue data from lower-limb tendons, however this may lead to inaccurate conclusions when assessing overuse injury risk at the wrist. Therefore, the purpose of this study was to characterize the fatigue behaviour of FDP and FDS tendons. We found that similar to other tendons, cyclically loaded FDP and FDS tendons illustrated a logarithmic relationship between applied stress and fatigue life, however the exact parameters of the FDP/FDS stress-fatigue life relationship were unique and may improve the accuracy of current carpal tunnel syndrome and trigger finger WMSD evaluation tools. We also observed that creep and damage rate had the strongest correlations with fatigue life, suggesting that these metrics may represent promising future directions for WMSD risk evaluation.

Subsynovial connective tissue development in the rabbit carpal tunnel

The carpal tunnel contains the digital flexor tendons and the median nerve, which are embedded in a unique network of fibrovascular interconnected subsynovial connective tissue (SSCT). Fibrous hypertrophy of the SSCT and subsequent adaptations in mechanical response are found in patients with carpal tunnel syndrome (CTS), but not much is known about the development of the SSCT. This observational study describes the morphological development of SSCT using histology and ultramicroscopy in an animal model at four time points between late-term fetuses through adulthood. A transition is seen between 3 days and 6 weeks post-partum from a dense solid SSCT matrix to a complex multilayered structure connected with collagenous fibrils. These preliminary data show a developmental pattern that matches an adaptive response of the SSCT to loading and motion. Understanding the anatomical development aids in recognizing the pathophysiology of CTS and supports research on new therapeutic approaches.