Outline

The work and experiments of the author and his colleagues, including surgeons, histophysiologists, and biomechanical engineers, have convinced them that current understanding about tendon physiology is to be moved ahead. After carrying out in vivo dissections and video-recording of the tissues existing between the skin and tendons in carpal tunnel and forearm areas, the author and his colleagues questioned some basic mechanics of the tendon sliding. This chapter presents a new approach of morphological studies, and the author attempts to explain the existence of the multimicrovacuolar collagenous dynamic absorbing system. This system enables easy and extensive sliding of the tendons without interference from surrounding tissues, facilitating tissue interdependence. The multimicrovacuolar collagenous dynamic absorbing system, which initially seems chaotic and complex, is based on the vacuolar concept, which is found in many parts of the human body, and is structurally composed of multiple interconnected fibrous tissues organized to facilitate functional adaptation of the tendon during movement.

For many years, the scientific explanations concerning the natural mechanism of flexor tendon mobility in the fingers were limited to the notion of a virtual space or the existence of loose connective tissue organized in layers, but the biomechanical foundations for these theories were vague.¹^–³

The biomechanical characteristics and histological findings of the structures around the tendons are sometimes confusing, and the roles and the definitions of the paratenon, mesotendon, peritendon, and sheaths and have largely varied among authors and their described surgical procedures ⁴^–⁹ (Figure 43-1).

Figure 43-1 When the tendon moves, the movement is barely discernible. There is an absorbing system surrounding the tendon. The capillary network around the tendon changes as the tendon moves. Vascular network observed during flexion (A), and during extension (B).

When surgical dissection is performed in vivo, visual magnification demonstrates the presence of a varied arrangement of tissue connections; a histological continuum with no clear separation between the skin, the hypodermis, the vessels, the aponeurosis, and the muscles. Present everywhere are structures that allow sliding to take place.

We present a view of physiology of tendon sliding in human tissues, based on microanatomical observations that we made with the aid of video recording and analysis. We present new hypotheses concerning the organization of these subcutaneous tissues.

Materials and Methods

In Vitro Study of the Paratenon

This study was carried out on 30 human upper limb biopsies of flexor digitorum superficialis (FDS) and profundus (FDP) with their surrounding sheaths, and 26 animal samples including the flexor carpi radialis from cattle where the organization is very similar to that of the human flexor profundus (Figure 43-2).

Figure 43-2 MVCAS shown under electron microscope. A, The flexor carpi radialis of cattle was harvested for observation. B, Preparation frozen-dried samples. C, Appearance of the sliding structure, MVCAS, under the electron microscope (×5). D, Findings under a higher power magnification. Tissues surrounding the tendon are made of microvacuoles (×25).

The preparation was treated with potassium bichromate, placed in formalin, and finally in caustic soda, allowing for a softer and more complete hydrolysis (Prof. J. P. Delage, INSERM Laboratories, Bordeaux, France). It was then frozen and freeze-dried under standard conditions for dehydration. Afterward, it was dissected under a binocular loupe at ×3.5 magnification. Samples were taken, given a gold-metallic finish, and then observed under the electron microscope. The INSERM Laboratories (Prof. Herbage, Lyon, France) helped us to analyze the chemical components of this connective tissue.

In Vivo Study of Digital Zones 3, 4, and 5 by Microanatomical Video Endoscopic Observation

The tendon gliding system was observed and recorded on video in 65 cases of tendon revascularization in Kleinert’s zones 3, 4, and 5 after releasing the tourniquet.

All of the patients gave their consent before surgery. Of the 65 cases, 57 procedures were forearm island reverse flaps. The remaining eight procedures were axillary flaps. Static and dynamic observations were carried out using an endoscope with an attached camera Tri CCD-HD-Image 1 22220055 Karl Storz endoscopy fiber optic camera and Xenon Nova 201315 light source at ×25 magnification.

Continuous sequences were captured on video during flexion of the digital flexors to allow for subsequent analysis.

Results

In Vivo Observations

Macroscopic Observations

When the flexor tendon moves, its movement is barely discernible in the palm. There is no dynamic repercussion of the movement on the skin surface. However, the flexor tendon excursion is at least 2 cm, without any hindrance and without displacing any of the neighboring tissues in the palmar area or along the common carpal sheath.

This suggests the existence of some sort of shock-absorbing system.

It is also clear from observing the behavior of the vessels of the common carpal sheath after revascularization and during flexion and extension, that there is apparent disorder and irregularity of shape of the microvascular network. There are surprisingly complex forms of vascular distribution, a finesse to the microvascular network, which is much more complex than a simplistic mechanistic distribution.¹⁰^–¹⁴

When we observed the area around the tendon, we noted an apparently circular longitudinal and peripheral vascularization, which seemed to represent a real continuity between the sheath and the tendon, and which was not interrupted despite the excursion and distension occurring during sliding and subsequent return to the original position.

At first sight, this is not incompatible with classical anatomical descriptions.

Ten-Fold Microscopic Examination

However, during flexion and extension of the tendon, tenfold microscopic examination of zones 3, 4, and 5 enabled us to observe vascular patterns in different planes of excursion and with different speeds of vessel progression due to modifications in the capillary network and depending on variations in tendon movements (Figure 43-3). Small vessels are subjected to deformation during movement but do not follow any logical or rational sequence. Some vessels progress quickly, while others move more slowly, and some overtake other vessels. The diameter of the vessel seems to be of no importance in this process. There is dynamic progression with no apparent order or proportionality.

Figure 43-3 A, Cold light variable magnification endoscope and 3 CCD camera. B, Intriguing vascular patterns that change in the capillary network depending on the direction of tendon movements during flexion and extension. C, Vessels are not all going at the same speed and are in different planes.

Very little research has been done to study this mechanical phenomenon, since the issue was considered by many to have been solved by the concept of a virtual space (i.e., the tendon slides in the carpal sheath like a bullet in a gun barrel without touching the sides, or rather, it slides in membranous or visceral layers or by stratification of different coaxial layers).

In vivo observation has rendered this concept unacceptable partly because it is surgically impossible to define a clear field of dissection between the paratenon and the tendon (Figure 43-4).

Figure 43-4 A, Traction on the paratendon during surgery. B, Searching for a space over the tendon surface. C, Network between the tendon and the surrounding tissues: the MVCAS. D, The movable, elastic sliding tissues, MVCAS.

At 10-fold microscopic examination (Figure 43-5), video observation at rest showed an inaccessible microanatomical arrangement, a tissue-continuity, and a gel-like tissue surrounding the tendon. We saw a glossy structure stretching across the tendon. Within this tissue, collagen fibers can be seen framing the vessels in a random fashion. We were confronted with the notion of global dynamics and continuous matter between the tendon and the surrounding tissue, radically opposing the classical descriptions of sliding structures based on the notion of tissue stratification and a virtual space between the tissue layers. Instead, we found total histological continuity. It therefore became necessary to investigate this tissue further in order to gain better knowledge of its properties and its different roles.