Forces in Joints, Skeletal Biology, Analysis of Bone Remodeling, Mechanical Properties of Bone, Fatigue and Fracture Resistance of Bone, Mechanical Adaptation of the Skeleton, Synovial Joint Mechanics, Mechanical Properties of Ligament and Tendon

Skeletal Tissue Mechanics

Screws placed into cancellous bone in orthopedic surgical applications, such as fixation of fractures of the femoral neck or the lumbar spine, can be subjected to high loads. Screw pullout is a possibility, especially if low density osteoporotic bone is encountered. The overall goal of this study was to determine how screw thread geometry, tapping, and cannulation affect the holding power of screws in cancellous bone and determine whether current designs achieve maximum purchase strength. Twelve types of commercially available cannulated and noncannulated cancellous bone screws were tested for pullout strength in rigid unicellular polyurethane foams of apparent densities and shear strengths within the range reported for human cancellous bone. The experimentally derived pullout strength was compared to a predicted shear failure force of the internal threads formed in the polyurethane foam. Screws embedded in porous materials pullout by shearing the internal threads in the porous material. Experimental pullout force was highly correlated to the predicted shear failure force (slope = 1.05, R2 = 0.947) demonstrating that it is controlled by the major diameter of the screw, the length of engagement of the thread, the shear strength of the material into which the screw is embedded, and a thread shape factor (TSF) which accounts for screw thread depth and pitch. The average TSF for cannulated screws was 17 percent lower than that of noncannulated cancellous screws, and the pullout force was correspondingly less. Increasing the TSF, a result of decreasing thread pitch or increasing thread depth, increases screw purchase strength in porous materials. Tapping was found to reduce pullout force by an average of 8 percent compared with nontapped holes (p = 0.0001). Tapping in porous materials decreases screw pullout strength because the removal of material by the tap enlarges hole volume by an average of 27 percent, in effect decreasing the depth and shear area of the internal threads in the porous material.

Factors affecting the pullout strength of cancellous bone screws.

Although it is known that the mechanical environment affects the fracture healing process, the optimal conditions for the different stages of healing have not been defined. In the present studies, the influence of applying a very short period of axial micromovement with defined characteristics to healing fractures has been studied both in simulated and clinical tibial fractures. The fracture healing process is seen to be acutely sensitive to small periods of daily strain applied axially within two weeks of fracture. There are boundaries of strain magnitude and force of application of applied movement that, if exceeded, inhibit the healing process. The application of appropriate applied strain to clinical tibial fractures at a time shortly after injury, when most patients would be very inactive, appears to enhance the healing process when using external skeletal fixation.

Controlled mechanical stimulation in the treatment of tibial fractures.

Diaphyseal fractures of the tibia in 80 patients were treated by external skeletal fixation using a unilateral frame, either in a fixed mode or in a mode which allowed the application of a small amount of predominantly axial micromovement. Patients were allocated to each regime by random selection. Fracture healing was assessed clinically, radiologically and by measurement of the mechanical stiffness of the fracture. Both clinical and mechanical healing were enhanced in the group subjected to micromovement, compared to those treated with frames in a fixed mode possessing an overall stiffness similar to that of others in common clinical use. The differences in healing time were statistically significant and independently related to the treatment method. There was no difference in complication rates between treatment groups.

/pdf/axial-movement-and-tibial-fractures-a-controlled-randomised-2f7bbs7uva.pdf

Axial movement and tibial fractures. A controlled randomised trial of treatment

Fracture of the long bones results in a repair process that has the potential to restore the anatomic morphology and mechanical integrity of the bone without scar tissue. The repair process can occur in two patterns. In the first, under conditions of rigid stabilization, direct osteonal remodeling of the fracture line can occur with little or no external callus, a process known as direct bone repair. The second pattern of repair involves bridging of the fragments with external callus and formation of bone in the fracture site by endochondral healing. This type of repair is known as indirect bone healing and occurs under less rigid interfragmentary stabilization. The rate of healing and the extent of callus in this type of repair can be modulated by the mechanical conditions at the fracture site. Applying cyclic interfragmentary micromotion for short periods has been shown to influence the repair process significantly, and characteristics of this stimulus influence the healing response observed. In the current study, a short term interfragmentary cyclic micromovement applied at a high strain rate induced a greater amount of periosteal callus than the same stimulus applied at a low strain rate. This high strain rate stimulus applied later in the healing period significantly inhibited the progress of healing. The beneficial effect of this particular biophysic stimulus early in the healing period may be related to the viscoelastic nature of the differentiating connective tissues in the early endochondral callus. In the early endochondral callus, high rates of movement induce a greater deformation of the fracture fragments because of the stiffening of the callus. Alternatively, the transduction pathway may involve streaming potentials as a result of the high movement rate.

Strain rate and timing of stimulation in mechanical modulation of fracture healing

Design and performance of a fracture monitoring transducer

Measurement of fracture movement in patients treated with unilateral external skeletal fixation.

Design and testing of external fixator bone screws.

A method for the measurement of the mechanical stiffness of fractures treated with unilateral external skeletal fixation is described. The procedure for obtaining fracture stiffness results is detailed, and results showing three distinct patterns of healing, which can be related to in vivo biological events, are presented.

J.L. Cunningham

Papers

Design and performance of a fracture monitoring transducer

Measurement of fracture movement in patients treated with unilateral external skeletal fixation.

Design and testing of external fixator bone screws.

The measurement of stiffness of fractures treated with external fixation.

Mechanical Measurements of Fracture Repair