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A Descriptive Study of the Equine Proximal Interphalangeal Joint Using Magnetic Resonance Imaging, Contrast Arthrography, and Arthroscopy

Take Home Message

The dorsal and palmar pouches of the proximal interphalangeal joint are described in depth using MRI, contrast arthrography, and arthroscopy. Arthroscopic approaches allow adequate access to the proximal interphalangeal joint and anatomy surrounding the joint is described.


The proximal interphalangeal (PIP) joint has the least amount of motion of all the joints of the equine distal limb and is challenging to access arthroscopically. The objective of this study was to assist in the treatment of diseases related to the PIP joint by describing its anatomy and outlining arthroscopic methods as there is very limited description of arthroscopy of the PIP joint in the current literature. This study done by Dr. Lacy Kamm together with Drs. Goodrich, Werpy, and McIlwraith described areas of the PIP joint that are arthroscopically accessible, defines the soft tissue structures that must be avoided during arthroscope and instrument placement, and investigates the differences between the fore and hind PIP joint.

Materials and Methods

Cadaver limbs were used to perform anatomic modeling, magnetic resonance imaging (MRI) with MRI-compatible needles, computed tomography (CT) with contrast arthrography, and arthroscopy of the PIP joint. CT contrast arthrography was performed on three fore and three hind limbs. Twenty-four limbs, 12 fore and 12 hind, were used for arthroscopy. Areas that could be viewed arthroscopically were measured, and two arthroscopic approaches to the dorsal joint pouch were compared.


Imaging revealed that in order to prevent penetration of the axial palmar/plantar ligaments, abaxial palmar/ plantar ligaments, straight sesamoidean ligament, and the branches of the superficial digital flexor tendon, the palmar/plantar pouch instrument and arthroscope portals should be placed dorsal to the neurovascular bundle and just proximal to the epicondyles of the proximal phalanx.

No significant difference in the joint volume was found between the fore limb and the hind (p=0.137), though the mean volume was smaller in the fore limb than in the hind limb (8.89cm3±2.1 [std dev] vs. 10.76cm3±1.5, respectively). The only significant difference in visualization between the fore and the hind limb was the arthroscope portal in the proximal dorsal approach resulted in more visualization abaxially in the fore limb than in the hind (p=0.050) when visualizing the cartilage on the same side as the arthroscope portal.

There was no significant difference in the amount of joint viewed when using the more proximal or distal approach to the dorsal joint pouch (p=0.586). The average perimeter visualized was 224.62° when the proximal dorsal approach was used. This was only 62.4% of the total perimeter of the joint.


The dorsal and palmar/plantar joint pouches allowed for adequate arthroscopic visibility of the axial portions of the articular surface of the proximal and middle phalanx (P1 and P2). The abaxial portions of the articular surface were difficult to view due to the narrowing of the joint pouches abaxially. Palmar/plantar portals were placed dorsal to the neurovascular bundle to prevent injury of tendons and ligaments.

The one statistically significant difference between the fore and hind PIP joints was that the arthroscope could allow visualization more abaxial on the ipsilateral side of dorsal joint pouch in the fore limb compared to the hind limb. The cause for this is unknown as the soft tissues and bones of the PIP joint appeared very similar between the fore and hind on CT and MRI and the joint volumes were not significantly different on contrast arthrography CT. One possibility for the significant difference is that the hind pastern is more likely to stay in flexion due to the stay apparatus. Flexion of the joint causes the dorsal aspect to be more restricted and may impede abaxial motion of the arthroscope.
Concerning the dorsal joint, no significant difference in visualization was found when the arthroscope was placed at the level of the PIP joint versus 1.5 cm proximal to the joint (p=0.586). The more proximal arthroscopic approach in this study was better to use as it allowed for easy visualization of the medial and lateral aspects of the joint and was not prone to the accidental exiting of the arthroscope or instruments that can happen with the distal approach when the ipsilateral side was visualized. It was found that if the arthroscope portal was placed too far proximally it can limit "the ability to view the joint space, because the arthroscope is held against the articular surface of the distal end of the proximal phalanx by the joint capsule." Therefore, it is important to place the arthroscope up to 1.5 cm proximal to the joint surface, but no more proximal than this to allow for maximal manipulation of the arthroscope.


Funding for this study was provided by the John H. Venable Grant at the College of Veterinary Medicine and Biomedical Sciences at Colorado State University.


  1. Schneider et al. 1994
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