Load sharing is the distribution of mechanical force across multiple structures so that no single structure bears the full burden. In the human body, bones, muscles, tendons, ligaments, and cartilage all share the forces generated by movement and gravity. In orthopedic medicine, load sharing describes a treatment philosophy where a fractured bone and its surgical hardware split the work of supporting force, rather than the hardware carrying it all.
How Your Body Shares Load Naturally
Your musculoskeletal system is a network of load-sharing tissues. Bones are the primary load-bearing structures, resisting gravity and maintaining the body’s shape. But every time you move or exert force, adjacent tissues pick up part of the load. Cartilage cushions joints, spinal discs distribute compressive force, ligaments hold bones together at joints, and tendons transmit muscle force to the skeleton. No single tissue type handles everything alone.
Muscles generate force through contraction, then transmit that force through tendons to bone. Ligaments and connective tissues contribute passively, stabilizing joints while the active muscle contraction does the heavy lifting. This cooperation means the load of any given movement gets spread across a chain of tissues rather than concentrated in one spot.
The spine is a good example. About 80% of spinal loading passes through the anterior and middle columns (the vertebral bodies and discs), while the posterior elements handle the remaining portion. The vertebral body accounts for roughly 90% of the surface area between vertebrae, compared to just 10% for the posterior structures. Spinal discs behave like pressure vessels, transmitting force radially and uniformly, which allows them to withstand the large compressive forces that come from muscular effort.
Load Sharing in the Knee
The knee joint splits its load between the medial (inner) and lateral (outer) compartments, but not equally. In healthy knees during walking, the medial compartment typically carries somewhere between 53% and 100% of the total load, depending on the individual and the phase of the stride. During the first half of the stance phase, healthy subjects average about 66% of the load on the medial side.
This balance shifts in people with knee osteoarthritis. In affected knees, the medial compartment bears roughly 74% of the load during early stance and up to 90% during late stance, compared to 66% and 82% in healthy controls. That uneven distribution helps explain why osteoarthritis so often damages the inner knee first. When load sharing breaks down and one compartment takes a disproportionate share, the cartilage and bone in that area deteriorate faster.
How Bone Responds to Load
Bone is not static. It constantly remodels in response to the forces placed on it, a principle known as Wolff’s law. When bone experiences repeated loading, cells convert that mechanical signal into a biochemical response through a process called mechanotransduction. The key steps include mechanocoupling (sensing the force), biochemical coupling (translating it into chemical signals), signal transmission, and cell response (building or removing bone tissue).
Under increasing loads, bone adapts by expanding its cross-sectional area, increasing mineral density near joints, and reorganizing its internal scaffolding of tiny struts called trabeculae. These changes make the bone better at handling force. However, there is a downside: while bone adapts well, the cartilage covering joint surfaces does not keep pace. The geometric changes in bone that help it cope with load can actually accelerate cartilage breakdown, setting the stage for osteoarthritis.
This is also why load sharing matters in fracture treatment. When surgical hardware carries all the force (load bearing), the bone underneath gets “stress shielded,” receiving less mechanical stimulation than it needs to maintain density. Load-sharing hardware avoids this problem by letting the healing bone carry part of the load, encouraging it to remodel and strengthen naturally.
Load Sharing vs. Load Bearing in Fracture Repair
Orthopedic surgeons choose between two approaches when stabilizing a fracture with plates and screws: load bearing and load sharing. The distinction comes down to how much work the bone itself can still do.
Load-bearing hardware uses large, rigid plates that carry all the forces at the fracture site. This provides absolute stability and is necessary when bone fragments are too unstable or too scattered to contribute any structural support, as in comminuted (multi-fragment) fractures or fractures with significant bone loss.
Load-sharing hardware uses smaller plates that provide relative stability. The fractured bone on either side of the break acts as a buttress, carrying a meaningful portion of the force. This approach works for simpler fracture patterns where enough bone stock remains intact to share the mechanical work. A well-known example is the Champy technique for mandible fractures, which uses small miniplates placed along the jaw’s natural lines of stress. Because the bone itself still contributes to stability, the hardware can be lighter and less invasive.
The critical distinction: load sharing cannot be used when the fracture site lacks bony support. If the bone is too fragmented or a segment is missing entirely, there is nothing for the hardware to share the load with, and a load-bearing construct is required.
The McCormack Load Sharing Classification
For spinal fractures, surgeons use a scoring system developed by McCormack to predict whether posterior hardware alone will hold up or whether additional anterior support is needed. The classification scores three features of the fracture on a scale of 1 to 3 points each, for a total between 3 and 9.
- Sagittal collapse: how much the vertebral body has lost height (less than 30%, 30 to 60%, or more than 60%)
- Fragment displacement: how far bone fragments have shifted (1 mm or less, about 2 mm, or more than 2 mm)
- Correction of deformity: how much angular correction is needed (3 degrees or less, up to 9 degrees, or 10 degrees or more)
A higher score means the fractured vertebra can share less of the spinal load, increasing the risk that posterior-only hardware will fail. Scores of 7 or above have traditionally been considered a warning that the hardware is doing too much of the work and additional structural support is needed.
The evidence on this scoring system is mixed. A review of 21 studies found that 12 (57%) showed no significant relationship between the score and hardware failure. Nine studies did find that higher scores predicted implant failure or loss of correction. Among those nine, patients with high scores experienced hardware failure at notably higher rates: 63 out of 112 high-score patients versus 13 out of 99 low-score patients. One study specifically found that a score of 8 correlated with a high incidence of implant failure when only a single level of posterior hardware was used. The classification remains useful as one factor in surgical planning, but it does not reliably predict outcomes on its own.
Load Sharing in Prosthetic Limbs
For people with lower-limb amputations, load sharing is a central challenge in prosthetic socket design. The socket is the interface between the residual limb and the prosthesis, and how it distributes pressure directly affects comfort, skin health, and function.
Two main design philosophies exist. The patellar tendon bearing (PTB) socket concentrates force on the patellar tendon, an area considered highly tolerant of pressure. This creates a focused load path through a specific region. The total surface bearing (TSB) socket takes the opposite approach, distributing load as evenly as possible across the entire residual limb to minimize peak pressure at any single point.
Pressure mapping confirms these differences in practice. PTB users show loading concentrated at the patellar tendon, the front of the shinbone, and the outer calf muscle region. TSB users typically show even distribution throughout the socket, though misalignment between the socket and prosthetic foot can disrupt this pattern. Prosthetists fine-tune the fit by adjusting socket shape, liner material, alignment, and component selection, relying heavily on the wearer’s subjective feedback. What constitutes acceptable levels of pressure and shear stress at the skin surface is still not well defined, which partly explains why skin breakdown and infections remain common problems for prosthesis users.