Classification of Musculoskeletal Tissue Failure

Jondy Cohen, M.D.

Bone, like all solids can break. Physicians call breaks in bone "fractures". While most people are aware that normal bones can fracture under large forces, you may not be aware that abnormal bones can break in response to normal forces. Fractures of abnormal bone occurring under a normal load are called "pathologic fractures". Examples of pathologic fractures include breaks that occur through bone diseased by infection, tumor, or osteoporosis. Such abnormal bone is weaker than normal bone requiring less force to fracture. When a patient reports breaking a bone with relatively little force physicians must consider pathologic fracture as a possibility.

Soft tissues (tendon, ligament, and muscle) can also fail under normal loads. A strong similarity exists between injuries of hard tissue (fractures of bone) and injuries of soft tissue (tearing of muscle, tendon, & ligament). Although soft tissue injuries are not commonly categorized in this manner, we find the distinction between traumatic and pathologic fractures of bone useful in categorizing injuries of soft tissue (muscle, tendon, ligament). We propose a revised scheme for classifying both bone and soft tissue musculoskeletal injuries based on the amount of force causing the failure and whether the involved tissue has normal strength.

In our office orthopaedic practice we often see three types of injuries to musculoskeletal tissue which we refer to as acute, chronic, and pathologic. These categories differ in the quality of tissue involved, the strength of the force required to cause the injury (high- or low-energy) [For simplicity we categorized trauma to tissues as either high- or low-energy. High-energy trauma will cause failure of normal tissue. Low-energy trauma is of insufficient energy to cause failure to normal tissue.] ,and the methods of treatment required to heal the injury.

High-energy trauma results in acute injuries. Such an injury has no prodromal [occurring before the incident] pain. The pain usually starts suddenly and decreases over time. Fractures of bone and lacerations of tendons are examples of these injuries.

Low energy forces often cause microscopic injuries of normal tissue such as microscopic tears of the collagen fibrils that make up ligaments and tendons, or microscopic fractures of bone. These injuries are so small that they do not interfere with the normal function of the involved tissue. Since no pain or loss of strength results from these small lesions, the patient doesn't even know it's happened. Usually tissue repairs itself from microscopic tears of collagen fibrils or microscopic fractures of bone.

Chronic injuries result from low-energy trauma. Sometimes a low energy force causes a microscopic tear or fracture and before the body has time to repair the insignificant injury, another low energy force to the same area occurs. Each successive low energy traumatic event extends the tear or fracture. Eventually, what started as an asymptomatic microscopic injury slowly becomes a clinically evident macroscopic injury. Normal tissue becomes increasingly abnormal and prone to failure. In this way repetitive low energy trauma leads to symptomatic macroscopic tears or fractures. Pain in such cases usually starts slowly and builds with successive insults to tissues that fail slowly. Maladies of this type are often well suited to office treatment since most respond readily to non-operative regimes. Treatment for chronic injuries includes removing or altering the offending repetitive forces and restoring normal tissue strength and tension. Plantar fasciitis, tennis elbow, Achilles tendonitis and stress fractures of bone are all chronic injuries.

Pathologic injuries are also a result of low-energy trauma. This injury subtly differs from the chronic injury mentioned in the preceding paragraph in that the cause of the tissue failure is unrelated to the stress on the tissues. Examples include fractures through a bony cyst or tumor, or fractures through osteoporotic bone. The difference between chronic and pathologic injuries is subtle yet important. Pathologic failures occur in tissue already weakened by underlying disease. Chronic injuries occur in normal tissue that becomes weakened by the repetitive nature of the trauma.

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