Morphology and classification of mechanoreceptors in lateral ligaments of the human ankle
Mechanoreceptors have been identified in both human and animal joints [
17‐
19]. In the cat knee, the mechanoreceptors were identified in joint capsule as well as in ligament parenchyma [
17]. In human, the research of mechanoreceptors has been focused on large joints such as the knee and hip joint, only a few research focused on the collateral ligaments of ankles. Research on the morphological description and classification of mechanoreceptors in the ankle collateral ligaments have also been performed long time ago [
20,
21]. New emerging technologies have produces different staining techniques, the mechanisms of classification, the morphological variation, and the integrity of neuroanatomy and electrophysiology. In this study, we used gold-chloride staining methods due to this method is a cost-effective methods, yet providing a clear view of mechanoreceptors in the ligament.
However, the injury of the lateral ligaments of the ankle is very common in clinic, taking up about 16 % of the sports injuries in emergency [
22]. In the US, the incidence rate is one in ten thousand every day [
23], 10 % of the patients developed instability of the ankle joint, repeated sprain, and even osteoarthritis after injury. Moreover, the previous research focused on the ankle anatomy and biomechanics, neglecting the proprioceptive function, due to the limitation of morphological and neurophysiological evidence. In recent years, the ankle disorders, especially the proprioceptive loss and restore in ankle instability, have attracted more research interests in orthopedics.
The staining technique is the key to success of the studies. For a long time, the immunohistochemistry and silver staining have been applied to the studies on morphology of mechanoreceptors. To avoid the high cost and relatively complicated procedures, we compared different staining methods in the literatures and chose modified O’Connor and Gonzales gold-chloride staining method. This staining method can clearly demonstrate the morphology of mechanoreceptors, suggestive of a better way of staining nerve endings.
Mechanoreceptors have diverse morphology, in addition to the typical four types. The classification of mechanoreceptors was according to Freeman and Wyke [
17], which was widely accepted, as it precisely described the morphological features and functions of major mechanoreceptors in category. In our study, all types of mechanoreceptors in collateral ligaments were found one to three times smaller than described (Table
1). Moreover, the morphology and configuration of receptors in this study were different from typical four types reported in the literatures. For an instance, the Ruffini in human ankle ligaments are spindle shaped, differing from the widely accepted spherical shape. In addition, capillaries were found in some nerve fibers of Ruffini in this study, which suggested that Ruffini were closely related to blood circulation. Broom-shaped and botryoidal Pacini have not been reported before. Concordant with some reports [
24], mechanoreceptors with transitional morphology and structure were found in this study. This may be caused by variation of classic types or possible new types. The diversity of morphology suggests the complication of proprioceptive function, which requires further study.
The different morphologies of mechanoreceptors may be caused by the different subjects, in addition to staining technique. Animal joint capsule receptor morphology may be different from the human. As mentioned before, Freeman and Wyke observed animals like cat in their research, mechanoreceptors were larger than human beings in their studies. This indicates that during evolution, the proprioceptive function of human beings may have degenerated. The ankle activities of human beings are more complicated and take more proportion of the body weight, while animals that walk with four limbs shared fewer loads on hind ankles. This may result in the diversity of mechanoreceptors in human ankle ligaments. However, more evidences and research are required to prove this assumption. Another reason for the different morphologies can be that we used collateral ligaments of the ankle, they differ from anterior or posterior cruciate ligament in other studies. Ligaments in different locations may lead to difference in mechanoreceptor morphology.
The distribution and the proprioceptive function of mechanoreceptors
As a result from the controversy of the morphology and classification of mechanoreceptors, the amount and distribution of each type in the ligaments are disputed. It was reported that the types I and II mainly located in the joint capsule, rarely in ligaments. Michelson and Hutchins [
19] failed to find type IV in human ankle ligaments, which was observed in cat ankle ligaments by different researchers [
25]. Recently, Moraes identified a bunch of free nerve endings in lateral ligaments of the human ankle [
18]. Some researchers failed to identify the Pacini in joints [
24]. However, all typical four types of mechanoreceptors were observed in this study.
Concordant with most of the reports [
17,
19,
25], the Pacini were found to have the largest amount and widest distribution in this study. They largely exist in the synovial tissue at both ends of ligaments and the tissue intervals branching into the ligaments, accompanied by nerve vessel bundles. The anatomic structures were assumed to form a tension-compression device [
17]. The longitudinal tension of the ligament can compress the space of the fibrous septum, stimulating the mechanoreceptors. In this study, the long axis of the flat cells annularly wrapped Pacini was found parallel to the long axis of ligaments. This may provide an evidence for the tension-compression assumption. Although the Pacini morphology has been accepted by the majority of researchers, the distribution is still disputed. Some believed that the Pacini only existed in the loose connective tissues at both ends of the ligaments and the tissue intervals [
17]. In this study, the Pacini were identified in the whole ligament parenchyma, in addition to the synovial tissues at the distal and proximal parts of ligaments.
Statistics showed that the amount of mechanoreceptors in the distal, middle, and proximal segments of the collateral ligament had no significant difference. However, if the mechanoreceptors in synovial membrane were counted in, the results would be different. The same situation happened to the type III, which existed more in synovial membrane than in the ligament parenchyma. As a result, the statistics showed less of type III than reported [
18,
19], even less than type II.
In this study, we focused on the morphology, classification, and distribution of mechanoreceptors in collateral ligaments of the human ankle. The results could provide morphological evidence for the proprioceptive mechanoreceptors in the ligaments, which could help with the treatment of lateral ankle sprain and chronic ankle instability. Further research of the function of mechanoreceptors may lead to new strategies of treating ligament injuries of ankle joints.