Bone Fixator
Colorized scanning electron micrograph shows bone cells attaching to a new type of bone cement made of calcium phosphate. Credit: H.Xu/American Dental Association Foundation.

Factors Influencing Successful Fracture Healing (2/3)

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It was shown by Kenwright et al [13] that imposed interfragmentary motion has different effects at different stages of healing. Early fracture IFM (first 4-6 weeks) promotes high amounts of callus formation and helps healing [8]. However, once weight-bearing period starts, such IFM can delay fracture union [14] and therefore the fixation could be stiffened up. From the literature available to date, based on animals, clinical experience and modelling it can be concluded that the tolerable axial IFM for long bone ranges from 0.2 mm - 2 mm. In most cases displacements greater than 2 mm led to delayed union or non-union [14]. McKellop et al [15] have shown that during natural healing of tibia fractures, the elastic IFM ranges from 0.5 mm - 1.9 mm and also angulations or rotations from 0.7 to 1.2 degrees. Wolf et al [3] have found the optimum IFM of 0.5 mm for fractures with a gap size of 3 mm in their study on sheep diaphysal osteotomies. Claes et al [8] have shown that the size of the fracture gap plays an important role in determining optimum amplitude of axial displacement. Therefore, an interfragmentary strain (IFS) could be a better quantifying measure. Perren et al [16] have defined IFS as the IFM divided by the gap size. The reduction of the IFS from 45 % to 5 % during normal healing process was observed by Claes et al and Gardner et al [8, 17]. Claes et al [8] have explained such observation by splitting the healing into three stages based on the composition and the amount of callus formed. It was observed that as the bone healed, callus strength was increasing and the tolerance to strain was decreasing. Those results agree with bony tissue mechanical properties [18-20], presented in Table 1.

Mechanical properties of human bone tissues.
Table 1. Mechanical properties of human bone tissues.

These data suggest that granulated tissue, which forms in the early stage of healing, can tolerate high rates of strain of up to 100 %. However once fibrous tissues, tendon and cartilage surround the fracture, strain tolerance significantly decreases to 10 %, and once the lamellar bone is formed, the maximum strain tolerance further decreases to 2 %. In addition, Claes et al [8] have looked into the quality of the newly laid bone and concluded that despite larger IFM-promoted higher rates of callus formation, tissue quality suffered.

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