Ligament and Tendon Enthesis: Anatomy and Mechanics - Structural Interfaces and Attachments in Biology - page 80

ligament and decreasing into the midsubstance tissue. In other words, the findings
suggest that insertion sites can undergo greater elongation before failure.
The reason for differences in the elongation of the ligament substance and the
ligament strain may be due to the changes in tissue composition that occur across
the insertion. The tissue changes from the ligament or tendon substance (primarily
collagen type I with some proteoglycans), to fibrocartilage (collagen types I, II, III,
and aggregan), and finally to bone (collagen type I). Histologically, a change in cell
shape and decrease in collagen orientation has been observed nearing the bony
insertion [ 10 ]. Particularly in ligaments such as the ACL, the insertion site is often
much wider than the midsubstance tissue [ 30 ], and thus collagen fibers are not able
to fully align under tension and achieve the stiffness found in the ligament
substance.
Recently, new techniques have been developed to better characterize the strain
distributions across ligaments and tendons. Spalazzi et al. [ 31 ] used ultrasound
elastography to examine the response of the ACL substance and insertion site under
tension, finding only tensile strain in the ligament substance, but both tensile and
compressive strain in the insertion. A nonuniform strain distribution was also
observed from the tissue substance to the bone, and strain was found to be highest
at the insertion site.
4.6 Repair of Ligament and Tendon Insertion Sites After Injury
Repair of the soft tissue-bone junction following avulsion injuries depends on the
location and severity of the injury. Some investigators advocate nonsurgical ther-
apy for nondisplaced or minimally displaced avulsion injuries [ 32 - 34 ]. Others
prefer reattaching the soft tissue end (tendon, ligament, or joint capsule) to the
denuded bone with transosseous nonabsorbable sutures, screws with toothed or
spiked washers, and staples [ 35 - 37 ]. Results of these procedures can vary greatly
and are dependent on the method used as well as the tissue type. For example,
Robertson and associates [ 38 ] showed that screw fixation with a plastic spiked
washer or soft tissue plate had initial loading capabilities superior to those with
staple and suture fixation. In terms of the ultimate load at failure during tensile test,
the screw with the spiked washer proved to be optimal for capsular tissue or thicker
tendons, while the screw with the soft tissue plate was best for thinner tendinous
tissue. In the past several years, improvement of the arthroscopic technique has led
to the development of different types of suture anchors, which have become one of
the most commonly used tools for arthroscopic repair of the rotator cuff and glenoid
labrum in the shoulder [ 39 , 40 ]. Suture anchors consist of metallic or bioabsorbable
anchors that are inserted into bone with attached eyelets to pass sutures through.
These devices provide firm fixation of soft tissue to bone, which is required for
tendon-bone healing. Recently, suture anchors have also been used for hip arthros-
copy, especially for tears of the acetabular labrum [ 41 ].