Dr. Kevin Yip

Dr Kevin Yip
Orthopaedic Surgeon
MBBS(UK), FRCS(EDIN), FAM(SING), FHKCOS(ORTHO)

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Periarticular Ligamentous Tissue: A Biochemical and Physiologic Assessment-Structure

Structure

The histology of periarticular tendons and ligaments is a much-neglected area of investigation. Although many histology texts combine these two tissues as “dense, regular connective tissue”, certain ligaments and tendons are sufficiently characteristic to be distinguishable from each other based upon their histological appearance .

The variables considered are collagen bundle width, cell morphology and size, as well as “crimp.” Crimp is a feature of tendons and ligaments; it represents a regular sinusoidal pattern in the matrix. The periodicity and amplitude of crimp appear to be structure-specific features, and they are best evaluated under polarized light.

A simple functional explanation for this accordionlike pattern in the matrix is that it provides a “buffer” in which slight longitudinal elongation may occur without fibrous damage. It also provides a mechanism for control of tension and acts as a “shock absorber” along the length of the tissue. When physiologic mechanical limits of this crimp are exceeded, however, irreversible damage occurs and the physical properties of the tissue are changed .

Although tendons and ligaments have crimping within their fascicles, there appear to be differences in the crimp pattern between these two structures . In the canine anterior cruciate ligament (ACL) and patellar tendon, two patterns of crimping are noted. The centrally located fascicles in the ACL are either straight or undulated in a planar wave pattern, whereas those located at the periphery are arranged in a helical wave pattern. In the patellar tendon, all the fascicles are found to undulate in the helical wave pattern.

The following histological description refers to rabbit tendons and ligaments unless otherwise specified. Microscopic examination of patellar tendon sections that have been stained with hematoxylin and eosin or evaluated directly under polarized light reveals the presence of longitudinally oriented bundles of collagenous tissue. These bundles are approximately 20 micrometers (µm) in width and have the characteristic crimp pattern of regular connective tissue.

In patellar tendon, the crimp period is approximately 120 µm in length, with a corresponding amplitude of about 15 µm. On either side of the bundles or fascicles are spindle-shaped fibroblasts that are approximately 25 µm long.

They are aligned longitudinally. Cytoplasm is indistinct, and only nuclei can be seen. More cellular areas within areolar connective tissue are observed. These sites, which are actually investing layers of tissue, are called “peritendineum”  and have been previously described as a site of reserve cells . They also mark the site of nerve and blood supply to the tendon.

Rabbit Achilles tendon demonstrates cell morphology, cell size, bundle width, and crimp period that are similar to those of patellar tendon. The crimp amplitudes of these tissues are different, with the Achilles tendon having almost three times (40 µm) the wave height relative to the patellar tendon. This difference may be related to an increased margin of shock absorbance for the Achilles tendon.

Histological assessment of the ACL demonstrates longitudinally oriented bundles of collagen with a width of about 20 µm as seen in the patellar tendon. The crimp period in the ACL, however, is considerably shorter (45 to 60 µm), and the amplitude is less than 5 µm. Fibroblasts are located on either side of the collagenous bundles, but the ligament is considerably more cellular than the tendon.

ACL fibroblasts are round to ovoid and are substantially different in appearance from the fibroblasts in the patellar tendon. They measure about 5 to 8 µm in diameter and 12 to 15 µm in length. Cells are arranged longitudinally along the borders of the fascicles.

Like the patellar tendon, groups of cells, which concentrate in areolar connective tissue. These may be the ligamentous correlates to the peritendineum areas.

The medial collateral ligament (MCL) of the knee is notable for rod-shaped and spindle-shaped cells that are intermediate in length compared with patellar tendon and ACL cells. The MCL has cells that are 15 µm long and 25 µm wide. The crimp period measures approximately 45 µm, with a 10 µm amplitude, and the collagen bundle width is approximately 20 µm, as seen in the other structures discussed.

These substantial differences in morphology and ultrastructure may reflect the functional and environmental differences between these two periarticular ligaments, the ACL and MCL. The cellular morphological characteristics of the MCL are those of all fibroblasts, whereas the ACL cellular characteristics are similar to fibrocartilage cells. These observations lead to a series of profound and important questions concerning the differences in function, homeostasis, and repair between the ACL and MCL.

The biochemical parameters used to assess the constitutional properties of collagenous tissue include collagen structure and type, collagen reducible and nonreducible crosslink analysis, proteoglycan content, and glycoprotein. The value of each of these variables is related to its importance in the study of both soft tissue injury and healing and the response to exercise and the deleterious effects of immobilization.

A more complete understanding of these problems could improve various treatment modalities and place them on firm scientific ground. This is particularly the case because most investigations of tissue injury and healing involve skin, not tendons or ligaments.

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