James L. Cunningham

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

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.

Measuring stiffness can define healing of tibial fractures

We measured fracture stiffness in 212 patients with tibial fractures treated by external fixation. In the first 117 patients (group 1) the decision to remove the fixator and allow independent weight-bearing was made on clinical grounds. In the other 95 patients (group 2) the frames were removed when the fracture stiffness had reached 15 Nm/degree. In group 1 there were eight refractures and in group 2 there was none (p = 0.02, Fishers exact test). The time to independent weight-bearing was longer in group 1 (median 24 weeks) than in group 2 (21.7 weeks, p = 0.02). The greater precision of our objective measurement was associated with a reduction in refracture rate and in the time taken to achieve independent weight-bearing. We consider that a stiffness of 15 Nm/degree in the sagittal plane provides a useful definition of union of tibial fractures.

Tibial external fixation, weight bearing, and fracture movement.

Axial fracture movement and loading has been measured during weight bearing in 45 patients with tibial diaphyseal fractures treated with unilateral external skeletal fixation. Mean axial fracture displacement reached a maximum of 0.6 mm between seven and 12 weeks postfracture. Very little movement occurred during the first five weeks after fracture. A micromovement module attached to the fixator increased axial movement at the fracture site by 50% during walking. Weight bearing reached 75% of mean body weight by ten weeks after the fracture. Weight bearing was not decreased by any biofeedback mechanism. A randomized prospective clinical study of diaphyseal tibial fractures treated with external fixation showed a significant reduction in time to healing when micromovement was imposed. Controlled fracture site movement can be imposed very early after fixator frame application when mechanical stimulation may be most effective, and the active loading by the patient is least.

Fibula and its ligaments in load transmission and ankle joint stability

A study was made of the role of the fibula in weightbearing and its contribution to ankle joint stability in 10 anatomic specimen lower limbs. On axial loading of the lower limb, the fibula was found to take an average of 17% of a 1500 N axial load. The proportion of the load carried by the fibula increased with the total loading. It also increased when the line of load was displaced laterally and when the ankle joint was in dorsiflexion and decreased when the line of loading shifted medially or the joint was plantar flexed. With loading, the lateral malleolus migrated distally relative to the medial malleolus, except after fibular osteotomy, when it migrated proximally. There was an approximately inverse relationship between proportional fibular loading and distal fibular migration. Cutting the inferior tibiofibular ligament reduced the proportional load in the fibula and increased its distal migration. The interosseous membrane modified the load distribution between the tibia and the fibula, with the distal fibula carrying a higher proportion of the axial load than did the proximal. Surgical repair of a ruptured inferior tibiofibular ligament, using either 1 or 2 screws, was associated with an abnormal pattern of load distribution and fibular displacement.

Controlled induction of a pseudarthrosis: a study using a rodent model.

Objectives This study aimed to test the hypothesis that under standardized mechanical and biologic conditions, the process of indirect bone repair in a rodent species could be manipulated to form a reproducible, atrophic, fibrous pseudarthrosis. Design The model used comprised a mid-diaphyseal, transverse osteotomy in the rat femur, stabilized via a precision miniature external fixator, a constant axial fixation stiffness being defined by a specific frame geometry. Main Outcome Measurements The repair process for both 0.5-mm and 3.0-mm gap osteotomies was characterized using radiography, dual-energy x-ray absorptiometry, histologic assessment of standardized longitudinal sections, and postmortem mechanical testing. Results Healing of the defect was highly reproducible, bone union being attained at around 5 weeks postoperatively with a 0.5-mm gap. Increasing the gap width to 3.0 mm resulted consistently in a pseudarthrosis. Conclusion These two reproducible patterns of repair can now be used to elucidate the underlying molecular mechanisms controlling the extent and progression of connective tissue differentiation in indirect bone repair without the additional variable of a nonstandardized mechanical environment.

Mechanical characteristics of antibiotic-laden bone cement.

We studied the mechanical characteristics of cement-antibiotic combinations in vitro. Palacos R was tested without antibiotics, with gentamicin alone and with gentamicin plus vancomycin or flucloxacillin. Palacos LV was studied only with gentamicin added. CMW I was studied with gentamicin added, with gentamicin plus vancomycin, and with gentamicin plus flucloxacillin. We performed four-point bending tests on beams of cement to establish bending strength and modulus, and compared the values to ISO standards. Density was also assessed. Palacos R was the strongest of the cements (bending strength 80 MPa). Palacos formulations (apart from Palacos LV) had a higher density and bending modulus than CMW 1. Statistical comparison of various cements with plain Palacos R showed lower density in 4 of the mixtures, and lower bending strength and modulus in 6 of the mixtures. Palacos R/gentamicin plus vancomycin and CMW 1/gentamicin plus vancomycin had bending strength slightly above minimum ISO standards, suggesting that the addition of vancomycin during cementmixing may compromise the outcome in revision surgery for sepsis.

Monitoring the Mechanical Properties of Healing Bone

Fracture healing is normally assessed through an interpretation of radiographs, clinical evaluation, including pain on weight bearing, and a manual assessment of the mobility of the fracture. These assessments are subjective and their accuracy in determining when a fracture has healed has been questioned. Viewed in mechanical terms, fracture healing represents a steady increase in strength and stiffness of a broken bone and it is only when these values are sufficiently high to support unrestricted weight bearing that a fracture can be said to be healed. Information on the rate of increase of the mechanical properties of a healing bone is therefore valuable in determining both the rate at which a fracture will heal and in helping to define an objective and measurable endpoint of healing. A number of techniques have been developed to quantify bone healing in mechanical terms and these are described and discussed in detail. Clinical studies, in which measurements of fracture stiffness have been used to identify a quantifiable end point of healing, compare different treatment methods, predictably determine whether a fracture will heal, and identify factors which most influence healing, are reviewed and discussed.

Accurate diagnosis of hip prosthesis loosening using a vibrational technique

OBJECTIVE To examine the potential role of vibration testing as a non-invasive method of diagnosing loosening of total hip replacements in the clinical setting. DESIGN Single blind cohort study in two hospitals. BACKGROUND Diagnosing loosening of total hip replacements is heavily dependent on investigative techniques that are unreliable. Previous studies into the use of vibration testing have produced conflicting results. METHODS Comparison of vibration testing and radiographs in patients with a total hip replacement experiencing hip pain symptomatic of loosening, with patients showing evidence of a secure prosthesis. RESULTS Vibration testing has a sensitivity of 80% and a specificity of 89%. The positive predictive value was 92% and the negative predictive value was 73%; it was unable to produce a definitive diagnosis in 8% of patients. When compared with radiographs from the same patients, vibration testing was shown to be 20% more sensitive and able to diagnose 13% more patients. CONCLUSIONS Vibration testing can deliver more accurate information on the stability of total hip replacements than radiographs in the clinical setting, despite being in the early stages of development. Relevance. This study shows that 70 more patients may be provided with the correct diagnosis and 46 fewer patients may be undiagnosed each year, when using vibrometry as opposed to radiographs. In view of the relative disparity between the level of development between the two techniques and the encouraging results hitherto presented, it is felt that by improving vibration testing it may supersede radiographs in the detection of prosthesis loosening.

Ultrasonic propagation in cortical bone mimics

Understanding the velocity and attenuation characteristics of ultrasonic waves in cortical bone and bone mimics is important for studies of osteoporosis and fractures. Three complementary approaches have been used to help understand the ultrasound propagation in cortical bone and bone mimics immersed in water, which is used to simulate the surrounding tissue in vivo. The approaches used were Lamb wave propagation analysis, experimental measurement and two-dimensional (2D) finite difference modelling. First, the water loading effects on the free plate Lamb modes in acrylic and human cortical bone plates were examined. This theoretical study revealed that both the S0 and S1 mode velocity curves are significantly changed in acrylic: mode jumping occurs between the S0 and S1 dispersion curves. However, in human cortical bone plates, only the S1 mode curve is significantly altered by water loading, with the S0 mode exhibiting a small deviation from the unloaded curve. The Lamb wave theory predictions for velocity and attenuation were then tested experimentally on acrylic plates using an axial transmission technique. Finally, 2D finite difference numerical simulations of the experimental measurements were performed. The predictions from Lamb wave theory do not correspond to the measured and simulated first arrival signal (FAS) velocity and attenuation results for acrylic and human cortical bone plates obtained using the axial transmission technique, except in very thin plates.

Modelling the effects of different fracture geometries and healing stages on ultrasound signal loss across a long bone fracture

The effect on the signal amplitude of ultrasonic waves propagating along cortical bone plates was modelled using a 2D Finite Difference code. Different healing stages, represented by modified fracture geometries were introduced to the plate model. A simple transverse and oblique fracture filled with water was introduced to simulate the inflammatory stage. Subsequently, a symmetric external callus surrounding a transverse fracture was modelled to represent an advanced stage of healing. In comparison to the baseline (intact plate) data, a large net loss in signal amplitude was produced for the simple transverse and oblique cases. Changing the geometry to an external callus with different mechanical properties caused the net loss in signal amplitude to reduce significantly. This relative change in signal amplitude as the geometry and mechanical properties of the fracture site change could potentially be used to monitor the healing process.