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 br anches of the tibial nerve in human cadavers, Our primary motivation w as 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 innervat e primarily the posterior joint capsule of the knee (Gardner, 1948, An at, Rec,, 101:109-130); the main branch corresponds anatomically to th e posterior articular nerve (PAN), which has been studied extensively in animals such as the cat, Materials and Methods: By using light micr oscopy, we counted the numbers of myelinated fibers in articular branc hes of the tibial nerve removed from eight cadavers, Unmyelinated fibe rs were counted in the same specimens by using electron microscopy, an d the percentage of unmyelinated fibers was calculated, Results: We fo und on average 2,280 myelinated fibers in the main articular branch an d 279 fibers in individual capsular ramifications, Myelinated fiber di ameter histograms showed either bimodal (peaks at 3-4 and 9 mu m) or u ninodal (peak at 3-4 mu m) distributions, depending on the specimen, T hese histograms were similar in appearance for both the individual cap sular 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 (ave rage percentage of unmyelinated fibers = 78.5%), Conclusions: The perc entage 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 j oint capsule; therefore, the best location for placement of a neural r ecording 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. (C) 1996 Wiley-Lis s, Inc.