Pathogenesis of Soft Tissue and Bone Repair
Boris A. Zelle and Freddie H. Fu
Musculoskeletal injuries usually result from supraphysiologic stresses that overwhelm the intrinsic stability of the musculoskeletal apparatus. The consequence is injury to the bone, tendon, muscle, ligaments, or a combination of these structures. The physiologic healing response varies among these tissues and is influenced by various intrinsic and extrinsic factors. Among these are the degree and anatomic location of the injury, the patient’s physiology, and the mode of treatment rendered. The aim of this chapter is to review the concept of soft tissue and bone healing and to describe the factors that influence the healing response.
Incision And Wound Healing
With regard to epithelial tissue, the surgical incision is considered to be a “controlled trauma.” Incision and wound healing begins immediately after surgery and progresses through four distinct phases: (1) the coagulation phase (Fig. 1-1), (2) the inflammatory phase, (3) the granulation phase (Fig. 1-2), and (4) the scar formation and maturation phase. Table 1-1 gives an approximate time frame for each of these phases with hallmarks of what each phase accomplishes. Wound healing requires a clean environment, good circulation, appropriate approximation of wound edges, and a balance of the cellular mechanisms that ensure a proper immune response in the wound environment. Wound healing occurs through scar formation. Many intrinsic factors (e.g., age, metabolic and circulatory disorders, patient physiology, and comorbidities) and extrinsic factors (e.g., nutrition, hydration, smoking, wound exposure, and wound management) will influence the healing response and formation of the scar.
TABLE 1-1
| Coagulation Phase (see Fig. 1-1) | Vasoconstriction, platelet aggregation, clot formation | Begins immediately and lasts minutes |
| Inflammatory Phase | Vasodilation, polymorphonuclear (PMN) leukocytes, phagocytes | At the edges of wounds, epidermis immediately begins thickening; within the first 48 hours entire wound is epithelialized; lasts hours |
| Granulation Phase | Fibroplasia, epithelialization, wound contraction | Fibroblasts appear in 2-3 days and are dominant cell by day 10 |
| Scar Formation/Maturation Phase (see Fig. 1-2) | Collagen synthesis; rarely regain full elasticity and strength | Lasts weeks to months and even up to 1 year |
Adapted from Browner BD, et al: Skeletal trauma—basic science, management, and reconstruction, ed 3, Philadelphia, 2003, Saunders.
Ligament Injuries And Healing
Ligament Anatomy and Function
Ligaments are anatomic structures of dense, fibrous connective tissue. They can be divided into two major subgroups: (1) ligaments connecting the elements of the bony skeleton (skeletal subgroup) and (2) ligaments connecting other organs, such as suspensory ligaments in the abdomen (visceral subgroup). The skeletal ligaments are the focus of this chapter. The nomenclature of the ligaments usually relates to their anatomic location and bony attachments (i.e., medial collateral, posterior talofibular), as well as their shape and function (i.e., triangular, cruciate, or deltoid ligament).
Structurally, ligaments contain rows of fibroblasts within parallel bundles of collagen fibers. Approximately two thirds of the wet weight of a ligament is water, whereas collagen fibers account for approximately 70% of the dry weight. More than 90% of the collagen in ligaments is type I collagen. Trace amounts of other collagens exist, such as type III, V, X, XII, and XIV.1 The primary structure of the type I collagen consists of a polypeptide chain with high concentrations of glycine, proline, and hydroxyproline. Almost two thirds of the primary structure of type I collagen consists of these three amino acids. Intermolecular forces cause three polypeptide chains to combine into a triple helical collagen molecule. This ropelike configuration imparts great tensile strength properties (Fig. 1-3). Within the ligament, the collagen fibrils are usually organized in a longitudinal pattern and are held in place by the extracellular matrix (see Fig. 1-1).2 Collagen fibers in the extracellular matrix are surrounded by water-soluble molecules, such as proteoglycans, glycosaminoglycans, and structural glycoproteins. Although these molecules represent only approximately 1% of the dry weight of ligaments, they are important for proper ligament formation and organization of the ligament meshwork. Their hydrophilic properties are crucial for the viscoelastic capacity of ligament tissue and ensure adequate tissue lubrication and proper gliding of the fibers. Moreover, proteoglycans couple adjacent collagen fibrils together and support the mechanical integrity of the ligaments.3
Ligament Injury
From the clinical standpoint, ligamentous injuries are classified into three grades.4 Grade I injuries include mild sprains. The structural integrity of the ligament is intact, although edema, swelling, and punctate ligament bleeding may be present. In grade II injuries, individual fibrils are torn, but the overall continuity of the ligament is maintained. Significant edema and bleeding is usually noted, and ligament stability is reduced. Grade III injuries are characterized by complete disruption of the ligament substance. Most ligamentous injuries can be diagnosed through a clinical examination and joint stability tests. Magnetic resonance imaging (MRI) represents the most commonly performed imaging study for diagnosing ligamentous injuries.
Multiple healing studies involving the medial collateral ligament (MCL) of the knee have been performed and have contributed to our knowledge of ligament healing. The healing phases of ligaments are traditionally divided by their morphologic appearance into an inflammatory phase (first days postinjury), a proliferative phase (1 to 6 weeks postinjury), and a remodeling phase (beginning at 7 weeks postinjury) (Table 1-2).5 It is important to appreciate that these three phases represent a continuum rather than distinct phases. The predominant cell types in the inflammatory phase are inflammatory cells and erythrocytes. As the ligament ruptures, its torn ends retract and have a ragged, “mop-end” appearance. The gap between these torn ends is filled with hematoma from ruptured capillaries. Histologically, the inflammatory reaction is characterized by increased vasodilation, capillary permeability, and migration of leukocytes. During the inflammatory phase, water and glycosaminoglycans are increased in the injured tissue. During the proliferative phase, a highly cellular scar develops, with fibroblasts as the dominating cell type. New collagen fibrils can be identified as early as 4 days after the injury. After approximately 2 weeks, the newly formed collagen fibrils bridge the gap between the torn ligament ends. However, the water content of the scar remains elevated, the collagen density remains low, and the collagen fibrils still appear less organized than in normal ligament tissue. During the remodeling phase, cellularity and vascularity decrease while collagen density increases. Moreover, the collagen arrangement becomes more organized along the axis of the ligament.
TABLE 1-2
| Inflammatory Phase | Vasodilation, fibrin clot formation, increased capillary permeability, and migration of leukocytes | Begins immediately and lasts minutes to hours |
| Proliferative Phase | Fibroblasts are the dominate cell type, collagen fibrils (as early as 4 days postinjury) | 1-6 wk postinjury |
| Remodeling Phase | Collagen synthesis and increased density; rarely regain full elasticity and strength | 7 wk up to 1 yr |
MCL healing studies in rabbits demonstrated that the remodeling phase is a long, ongoing process.6 At 10 months after ligament midsubstance injuries, the scar could be identified macroscopically and a significantly increased cross-sectional area of the scar was noticed. This scar tissue demonstrated an increased cellularity and highly organized scar tissue was not achieved, even at 10 months postinjury. Although the water concentration returned to normal value at 10 months, the glycosaminoglycan concentration of the scar tissue remained elevated and the collagen concentration remained lower. Despite a gradual increase throughout the healing phase, the collagen concentration plateaued at 70% of uninjured ligament tissue. In addition, the collagen types in the ligament scar varied from the normal tissue, with type III collagen being increased in the scar tissue.6
The healing response varies among the different ligaments. While MCL injuries have the potential to heal spontaneously, other ligament injuries, such as anterior cruciate ligament (ACL) injuries, rarely show a spontaneous healing response. Recent experimental studies in rabbits have demonstrated an increased expression of myofibroblasts and growth factor receptors in the injured MCL as compared with the injured ACL.7 Various reasons may account for the superior healing response of the MCL as compared with the ACL. It must be assumed that the high stress carried by the ACL prevents the ruptured ligament ends from having sufficient contact. In addition, the ACL is not embedded in a strong soft tissue envelope. Moreover, the ACL is an intraarticular structure; when it ruptures, the blood is diluted by the synovial fluid, preventing hematoma formation and hence initiation of the healing mechanism. Finally, it has been suggested that the synovial fluid is a hostile environment for soft tissue healing. Thus in ACL-deficient knees, the levels of proinflammatory cytokines are elevated, leading to a potentially unfavorable intraarticular microenvironment.8
Effect of Mobilization and Immobilization on Ligament Healing
An important aspect of the rehabilitation of patients with ligament injuries represents the timing of postinjury mobilization. Although aggressive mobilization obviously results in disruption of the scar tissue, prolonged immobilization may decrease the morphologic and biomechanical properties of the newly formed scar. It remains unclear what degree of immobilization is appropriate for healing ligaments.
The role of mobilization versus immobilization on ligament healing has been investigated in numerous animal studies.9–11 In an MCL healing study in rats, Vailas and associates11 compared the healing properties of the transected MCL across the following four groups: (1) surgical repair with 2 weeks of immobilization and 6 weeks of normal cage activity; (2) surgical repair with 2 weeks of immobilization and 6 weeks of treadmill exercise; (3) surgical repair with 8 weeks of immobilization; and (4) no surgical repair and no exercise. All animals were sacrificed at 8 weeks. The authors reported that the wet ligament weight, dry ligament weight, total collagen content of the ligament, and the ultimate load at failure of the ligament substance was lowest in the completely immobilized group and highest in the exercised group.11 In an MCL transsection model in the rabbit, Gomez and associates9 investigated the effect of continuous tension, as achieved by the implantation of a steel pin applying continuous stress on the healing MCL. At 12 weeks after MCL transection, the additional implantation of a tension pin resulted in a significantly decreased varus and valgus laxity, decreased cellularity of the scar tissue, and a more longitudinal alignment of the collagen fibers. These authors concluded that the application of controlled stress helped to augment the biochemical, morphologic, and biomechanical properties of the healing MCL.9 In a more recent study, Provenzano and associates10 investigated the effect of hind limb immobilization on the healing response of transected MCLs in a rat model. The authors reported significantly superior biomechanical ligament properties in the mobilized group. Microscopic analysis revealed abnormal scar formation and cell distribution in the immobilized group, as suggested by disoriented fiber bundles and discontinuities in the extracellular ligament matrix.10
These experimental data clearly emphasize the importance of stress and motion for the functional recovery of healing ligaments. However, the ideal amount of mobilization and immobilization during ligament healing is difficult to determine by animal studies, because animal studies are limited by the differing physiology and joint kinematics of animals. In addition, the amount of mobilization is difficult to control, and an exact titration of the stress cannot be performed with current in vivo models. Future clinical trials are necessary to determine the optimal amount of applied stress for the various ligament injuries.
Tendon Injuries And Healing
Tendon Anatomy and Function
Tendons are bands of dense, fibrous connective tissue interposed between muscles and bones. They transmit the forces created in the muscles to the bone, making joint motion possible. Some tendons may also connect two muscle bellies (e.g., digastrics, omohyoid). The gross tendon structure varies considerably from tendon to tendon, ranging from cylindrical rounded cords to flattened bands, called aponeuroses. The cross-sectional area of the more rounded tendons usually correlates with the isometric strength of the muscle from which they arise. The bony insertion site of the tendon is often accompanied by a small synovial bursa (e.g., subacromial bursa, pes anserinus, retrocalcaneal bursa). The tendon bursae are usually located in those anatomic sites where a bony prominence would otherwise compress the gliding tendon.
Microscopically, tendons and ligaments are similar. The tendon tissue is a complex composite of parallel collagen fibrils embedded in a matrix; cells are relatively rare, and fibroblasts represent the predominant cell type within the tendon; and the fibroblasts are arranged in parallel rows between the collagen fibrils (Fig. 1-4, A and B). The biochemical composition of ligaments and tendons are also very similar. Water is the major constituent of the wet tendon weight, whereas type I collagen accounts for approximately 70% to 80% and elastin for approximately 1% to 2% of the dry tendon substance. As in ligaments, other collagen types exist only in small amounts. The proteoglycans and glycosaminoglycans in the extracellular matrix play an important role for the viscoelastic properties and the tensile strength of the tendon. Their hydrophilic capacity provides the tendon with lubrication and facilitates gliding of the fibrils during tensile stress.
According to their envelope, tendons can be divided into tendons within a synovial sheath (i.e., sheathed tendons) and paratenon-covered tendons. In particular, tendons in the hand and foot are often enclosed in a synovial tendon sheath. The tendon sheath directs the path of the tendon and produces a synovial fluid, which allows tendon gliding and contributes to tendon nutrition. True tendon sheaths are only found in areas with an increased friction or sharp bending of the tendons (e.g., flexor tendons of the hand). A simple membranous thickening of the surrounding soft tissue, called the paratenon, usually surrounds tendons without a true synovial tendon sheath, such as the Achilles tendon. The paratenon is composed of loose fibrillar tissue. It also functions as an elastic sleeve and permits free movement of the tendon against the surrounding tissue, although it is not as efficient as a true tendon sheath.
Just like ligaments, tendons have a limited blood supply. The vascular supply of tendons has been described by injection studies, which demonstrated that tendons are usually surrounded by a network of blood vessels.12 Arteries supplying the tendon might come from the attached muscle, the bony insertion site, the paratenon, or the tendon sheath along the length of the tendon. However, there seems to be a difference between the nutrition of the sheathed tendons and the paratenon-covered tendons. The paratenon-covered tendons receive the majority of their blood supply from vessels in the paratenon. In sheathed tendons, the synovial sheath minimizes the vascular supply to the tendon substance, and avascular regions have been identified within the midsubstance of these tendons.12–14 Hence the diffusion of nutrients through the synovial fluid of sheathed tendons is critical for their homeostasis. Indeed, in sheathed tendons this process may be even more important than vascular perfusion. The digital flexor tendons, for example, receive up to 90% of their nutrition by diffusion.15 For this reason, sheathed tendons have also been referred to as avascular tendons, whereas the paratenon-covered tendons have been referred to as vascular tendons.
Tendon Injury
Tendon injuries may occur as a result of direct or indirect trauma (Fig. 1-5, A and B). Direct trauma includes contusions and lacerations, such as lacerations of the flexor tendons of the hand. Indirect tendon injuries are usually a consequence of tensile overload. Because most tendons can withstand higher tensile forces than their associated muscles or osseous insertion sites, avulsion fractures and ruptures at the myotendinous junctions are more likely than midsubstance ruptures. Midsubstance ruptures of the tendon after indirect trauma are usually associated with preexisting tendon degeneration. This has been supported by histologic investigations of ruptured Achilles tendons, which demonstrated increased tenocyte necrosis, loss of fiber structure, increased vascularity, decreased collagen content, and increased glycosaminoglycan content in previously ruptured tendons.16–18
Tendon Healing
The repair process in paratenon-covered tendons is also initiated by the influx of extrinsic inflammatory cells. As in ligaments, the healing of the ruptured tendon proceeds through an inflammatory phase, a proliferative phase, and a remodeling phase.19–21
