Gross anatomy, computed tomographic contrast tenography, and needle endoscopy of the equine medial digital flexor tendon sheath

Abstract

Objective: To investigate the computed tomography (CT) and gross anatomy of the equine medial digital flexor tendon sheath (MDFTS), and the endoscopic anatomy and approach to the MDFTS.

Study design: Ex vivo experimental study and one clinical case.

Sample population: Twelve clinically normal cadaveric hindlimbs.

Methods: Dissection, native and contrast CT scans were conducted to evaluate the anatomy of the MDFTS. Based on these findings, the portal locations for the endoscopic approaches were determined. Six hindlimbs were used in the pilot phase and another six limbs were selected for the main study. Endoscopic images were reviewed by three observers for the quality of visualization of intrasynovial structures during endoscopy.

Results: Intrasynovial structures that could consistently be identified during dissection and CT examination included the mesotenon in the proximal recess and two synovial plicae surrounding the medial digital flexor tendon. Communication between MDFTS and tarsal sheath varied among individual horses. Two portals were necessary to visualize the complete length of the MDFTS. Endoscopic entry was possible with both a needle scope and a conventional arthroscope; however, endoscopic examination was only feasible with the needle scope. The aforementioned intrasynovial structures could also be visualized endoscopically. One clinical case is presented with septic tenosynovitis due to a laceration with penetration of the MDFTS.

Conclusion: Endoscopic examination of the MDFTS is possible with the use of a needle scope.

Clinical significance: Endoscopic lavage of the MDFTS might be indicated in cases of septic tenosynovitis of the MDFTS and/or tarsal sheath.

FIGURE 1

Anatomical dissection of the medial aspect of a left hindlimb, with proximal to the left and caudal/plantar to the bottom of each image. (A) The proximal sheath‐like mesotenon (M) is seen attached to the caudoaxial aspect of the medial digital flexor tendon (MDFT*). (B) The flexor retinaculum (black arrow) crosses over the MDFT where it courses through the malleolar groove. The plantar fascia is partially incised distally (black arrowhead). Ch = chestnut. (C) Transverse folds (blue arrowhead) of the axial wall of the medial digital flexor tendon sheath (MDFTS) at the distal aspect of the medial plantar pouch of the talocrural joint. mM = medial malleolus. (D) Metal probe inserted underneath the synovial plica (blue arrow) coursing around the MDFTS, distal to the transected flexor retinaculum (black arrow). (E) A metal probe inserted in the synovial plica consistently seen just distal to the chestnut with the tip of the probe placed in the blind sack (yellow arrowhead). (F) Fusion of the MDFT with the lateral digital flexor tendon (LDFT).

FIGURE 2

(A) Medial aspect of a left hindlimb depicting the location for contrast injection into the tarsal sheath (TS) applied in the current study (black arrow) and the point of maximal palpable distention of the medial digital flexor tendon sheath (MDFTS, black arrowhead). (B) Frontal computed tomographic (CT) image showing the lateral digital flexor tendon (L) within the TS and the medial flexor tendon (MDFT) within the MDFTS (red arrowhead). (C) Transverse CT image of the proximal MDFTS showing the proximal mesotenon of the MDFT (red arrow). (D) Transverse CT image of the middle section of the MDFTS showing the outpouching of axial wall of MDFTS (blue arrow) into the distal aspect of the medial plantar pouch of the talocrural joint. (E) Transverse CT image of the distal MDFTS showing a filling defect within the MDFTS surrounding the MDFT (green arrow), corresponding with the synovial plica that forms an intrasynovial cul‐de‐sac just distal to the chestnut.

FIGURE 3

Endoscopic images of the medial digital flexor tendon sheath (MDFTS) obtained with a needle scope. (A) Synovial plica (SF) surrounding the medial digital flexor tendon (MDFT) distal to the flexor retinaculum. (B, C) Distal synovial plica (SF) forming a dorsal intrasynovial cul‐de‐sac (black arrow) distal to the chestnut. (D, E) Distal MDFTS showing the communication between the MDFTS and the tarsal sheath as fenestration in the synovial membrane (arrowheads). (F) Slightly more distally, the fusion of the MDFT and the lateral digital flexor tendon (LDFT) can be seen, as well as the continuation of the MDFTS into the tarsal sheath (*). (G) Endoscopic view looking proximal at the constriction of the MDFTS by the flexor retinaculum (FR) where it courses over the MDFTS in the malleolar groove. (H) Endoscopic visualization of the villi proliferation in the middle section of the MDFTS proximal to the FR and at the level of the transverse folds in the axial wall of the MDFTS. (I) Endoscopic visualization of the proximal MDFTS with the mesotenon (MT) attached to the axial wall.

FIGURE 4

Images illustrating a clinical case of a septic medial digital flexor tendon sheath (MDFTS) of the right hindlimb (A) due to a traumatic laceration (white arrowhead) at the medial aspect of the tarsus. (B) Transverse ultrasonographic image of the medial aspect of the tarsus showing the medial digital flexor tendon (*) within a distended MDFTS with increased heteroechogenic content. (C) Intraoperative clinical image showing the distal and proximal portals (white arrows) for endoscopic lavage of the MDFTS using a needle scope. (D) Endoscopic image of the tarsal sheath obtained with a conventional 4 mm arthroscope showing hyperemic synovium and moderate fibrinous deposition. (E, F) Images of the MDFT (*) within the MDFTS obtained by needle endoscopy showing moderate fibrinous deposition with thickening and hyperemia of the synovium. (G) Debridement of the MDFTS was performed with a small curette through the laceration.