Fiber type composition of articular branches of the tibial nerve at the knee joint in man.

Background: We examined the fiber type composition of the articular branches of the tibial nerve in human cadavers. Our primary motivation was to assess the suitability of these nerve branches for making neural recordings by using an interface, such as a nerve cuff electrode or a microelectrode array. Articular branches of the tibial nerve innervate primarily the posterior joint capsule of the knee (Gardner, 1948. Anat. Rec, 101:109-130); the main branch corresponds anatomically to the posterior articular nerve (PAN), which has been studied extensively in animals such as the cat.
Materials and Methods: By using light microscopy, we counted the numbers of myelinated fibers in articular branches of the tibial nerve removed from eight cadavers. Unmyelinated fibers were counted in the same specimens by using electron microscopy, and the percentage of unmyelinated fibers was calculated.
Results: We found on average 2,280 myelinated fibers in the main articular branch and 279 fibers in individual capsular ramifications. Myelinated fiber diameter histograms showed either bimodal (peaks at 3-4 and 9 microns) or unimodal (peak at 3-4 microns) distributions, depending on the specimen. These histograms were similar in appearance for both the individual capsular ramifications and the main articular branch of the tibial nerve. Numbers of unmyelinated fibers ranged from 4,176 to 5,200 in the main tibial nerve branch (average percentage of unmyelinated fibers = 69.6%) and from 750 to 2,250 in the individual capsular ramifications (average percentage of unmyelinated fibers = 78.5%).
Conclusions: The percentage of unmyelinated fibers is comparable to that found in articular nerves in other species. We discovered that the main articular branch of the tibial nerve contains a branch projecting distal to the knee joint capsule; therefore, the best location for placement of a neural recording interface to record from capsular afferents appears to be the well-defined ramifications of the articular branch that penetrate the joint capsule. Branches that contain only these ramifications are 0.5-1.0 mm in diameter and, on average, have 658 myelinated axons, which should be a sufficient number from which to record.