Medial Tibial Stress Syndrome (Shin Splints)
Medial tibial stress syndrome, commonly known as «shin splints», is an overuse injury of the tibia (shinbone). Contrary to popular belief, it is not an inflammatory condition.
Medial tibial stress syndrome (MTSS) is one of the most common causes of exercise induced leg pain. It goes by many names, including shin splints, shin splints syndrome, posterior tibial syndrome, medial tibial syndrome, and soleus syndrome. Currently, medial tibial stress syndrome is recognized as the most appropriate name for this condition (1-3).
MTSS presents as diffuse pain on the inside of the tibia during exercise, and the pain intensity is usually at its worst immediately after exercise. Some experience that the pain resolves during continued exertion, only to recur toward the end or after the activity. The pain is relieved with rest, but may last for days, especially following more vigorous exercise. For many athletes, MTSS is a recurring problem.
MTSS is a common injury in active individuals, especially in athletes participating in sports that involve a lot of running and jumping, including track and field, cross country running, football, basketball, dance and tennis. Studies show that as many as 14-20 % of athletes in running sports develop MTSS throughout one season (4). Military personnel are also particularly at risk, with 7-35 % of recruits developing MTSS during basic military training (5-7). Athletes most commonly develops MTSS when there has been a recent increase in training volume or intensity, or after starting up a new activity (8).
Not an inflammatory condition
Previously, MTSS was thought to be an inflammatory condition of the periosteum (the membrane that envelops the bone), caused by repetitive pulling by contractions of the deep muscles of the lower leg during exercise (1).
This theory was abandoned by the research community 20 years ago after several research papers made it clear that there are no inflammatory changes in the periosteum of patients with MTSS (9,10). Also, there are no other changes in the adjacent tendons or fascia that can explain the condition (11,12), and the muscles that were believed to create such a pull does not attach at the same area as the symptoms occur (13-17).
Still, even though researchers have long since abandoned this theory, is still frequently being used as an explanation for this condition by physicians and therapists around the world.
Bone stress reaction
The current theory of the etiology of MTSS is that it is a bone stress reaction caused by a local overload of the tibia. When the bone is subjected to high and repetitive loads without sufficient recovery, the activity in osteoclasts (cells that absorb the bone matrix) outpaces the osteoblasts (cells that produce the bone matrix). This results in impaired repair of the microscopic lesions in the cortical bone that results from repetitive strains during activity (3).
Over time, this reduces bone mineral density (BMD) of the affected area, and reduced cortical cross sectional area (CSA) (1,18-20), both of which have been found in athletes with MTSS (20,21). This, in turn, leads to a decrease in tibia's structural strength, and further reduces the tolerance to stress.
Research has shown that stresses experienced by the tibia during various activities are greatest in the same area as MTSS develops; in the lower two thirds of the inside of the tibia (1,20,22). The reason for this seems to be that this area is subjected to particularly large bending forces when we load the bone (23), which is less tolerated by the bone than compressive forces (24,25).
Illustration: Difference in bone mineral density (BMD) measurements between athletes with MTSS vs 1) age- and sex-matched hospital workers (left), and 2) age- and sex-matched athletes with comparable exercise regimens (right).
Athletes with MTSS had significantly lower BMD in the painful area than the non-injured groups (20). Illustration by Ken Fredin.