11 Biomechanical Assessment of Fracture Repair
The healing of a bone fracture is a continuous process in which the fracture ends reunite directly under stable mechanical conditions or form a stabilizing extra- and intramedullary callus under flexible mechanical fixation conditions. Typically during the process of healing, the mechanical stability of the healing bone steadily increases, eventually exceeding the mechanical stability of intact bone. The time for union varies anywhere between 8 weeks and 40 weeks depending on numerous factors including type of bone, kind of fracture, mode of fracture fixation, and most importantly on the definition of fracture union. Currently, there is no such thing as a gold standard for the definition of when a fracture is healed. Moreover, the different methods available for healing assessment do not correlate very well because they all assess different features of fracture healing.
Particularly in research settings, there is the need for objective measures of bone healing to monitor treatment and compare treatment methods. Although there is no consensus on when a fracture is actually healed, clinical studies as well as individual patient assessment require some sort of definition of a measurable end point of fracture healing. In orthopedic clinical studies, fracture healing often is one of the most important outcome variables and can be described by dichotomous (healed/not healed), multilevel ordinal (scoring system), or continuous variables. Healing is typically assessed at predefined time points at which the completion of the healing process is expected. On the other hand, a reliable indicator for the completion of the healing process (or the lack thereof) can also be of importance for the diagnosis of the individual patient. Such an indicator could guide decisions on cast or implant removal or could determine the need for further treatment or operation to achieve healing.
A fundamental problem in quantitatively assessing the healing status of a fracture is the fracture fixation system being used to stabilize the fracture. The purpose of fracture fixation or osteosynthesis is to stabilize the fracture fragments and enable the healing process but also to allow function and usage of the fractured extremity or body part. The mechanical stability of the fixation system always obscures the mechanical stability of the fracture itself. The challenge for biomechanical assessment of fracture healing therefore is to somehow eliminate or correct for the stabilization effect of the osteosynthesis system. The most appealing option is to temporarily remove the fracture fixation system. This, however, is restricted to external fixation systems such as plasters, casts, or perhaps external fixators. A second challenge is patient safety and compliance that limits any repeated measurement to noninvasive, inoffensive, and practical methodologies.
11.1 Direct Measurement Methods
Direct measurement methods directly assess a mechanical quality of the bone that changes during the course of the fracture healing process. It can be distinguished between methods assessing the structural integrity of the whole bone as compared to methods assessing local tissue properties. The structural integrity of healing bone is typically assessed by measuring the integral stiffness of the extremity by static or dynamic load application. Static loading deforms the extremity in relation to the amount of load applied. In dynamic deformation, the vibrational response of the extremity depends on the propagation of the induced oscillations, which are primarily determined by the overall mechanical integrity of the bone. Instead of measuring the overall integrity of the healing bone, direct measurements of mechanical properties can also focus on the site of fracture healing. Thus, measurement of local tissue properties at the fracture site directly reflects the mechanical changes of the healing tissue in the fracture gap and in the periosteal fracture callus.
11.1.1 Direct Measurement Methods: Stiffness
The most frequently assessed mechanical characteristic in fracture healing assessment is the overall stiffness of the fracture. Stiffness measurement requires simultaneous determination of the applied load and the resulting deflection generated by the load. The load can be applied manually by the investigator, by a defined weight that is put on the limb or by the patient itself. The amount of load is measured by a load cell. As a result of the load application, the limb deforms with most of the deformation occurring at the site of the fracture. The deformation needs to be measured as a function of the applied load. There are various methods to measure the deformation of the limb. One way is to directly measure the limb deformation. This can be done with goniometers attached to the skin surface, which measure the relative angulation between the distal and proximal fracture fragments. Alternatively, optical markers can be glued on the skin surface and their movement can be tracked by infrared or optical video capturing. If the fracture is stabilized by external fixation, the fixator pins or fixator bars can be used to track the deformation of the limb. Similarly, goniometers or optical trackers can be attached to the fixator frame and their movement can be measured as a function of the applied load. In patients who are treated by external fixators or by cast, the fixation device can be temporarily removed, providing direct access to the mechanical integrity of the fractured extremity. In the case of internal fixation with plates or intramedullary nails, direct measurement of mechanical integrity of the healing fracture is extremely challenging. The stabilizing effect of the osteosynthesis device completely conceals the instability of the fracture and renders deformation measurements on the skin surface useless. Therefore, the deformation needs to be measured directly at the fracture site with very high local resolution. One possibility is the use of instrumented implants, which employ biocompatible strain gauges directly attached to the osteosynthesis device 1 to measure the deformation of the implant. Alternatively, the deformation can be measured by radiography or fluoroscopic images acquired at the loaded and unloaded situation, respectively.
The loading mode for measuring the integrity of the healed bone can be bending, axial compression, or torsion. For the bending test, the bone to be tested is placed between two supports with the fracture located centrally between the supports. Three-point bending is created by loading the bone at the fracture site, either directly or via the external fixator frame (Fig. 11.1). This three-point bending setup has been frequently used in clinical studies to monitor the healing process in tibial fractures. 2 In most of these studies, a fracture stiffness of 15 Nm/degree was considered as an indicator for successful healing and guided the treatment decisions in further studies in which stiffness was measured. It has to be noted that this stiffness value was obtained in patients with casts or with external fixators, when the cast or fixator was temporarily removed or released.
Jargon Simplified: Composite Stiffness
If the stiffness of a healing bone has to be determined with the implant (fixator, plate) in place, the measured stiffness is composed of the stiffness of the implant and the stiffness of the fracture callus. In such a parallel composite construct, the deformation under an applied load is nearly identical for the implant and for the bone. However, the load is shared between both and is primarily transmitted by the stronger composite. Thus the composite stiffness during the early phase of healing is primarily determined by the stiffness of the implant. During the course of healing, an increasing amount of the load will be carried by the bone, reflecting its increase in stiffness. The stiffness of the healing bone can be calculated from the measured composite stiffness STotal and the stiffness of the implant SImplant using:
Accordingly, the load share (LS) is the percentage amount of load carried by the implant during external loading and is determined by
The stiffness of the implant has to be approximated by preceding calibration measurements.
Axial loading by the weight of the patient is a relatively straightforward procedure and can be performed with force plates or even electronic scales. In patients with external fixators, the deformation can be directly measured between individual fixator pins. For calculating the stiffness of the fracture, the composite stiffness of the fixator itself and the fracture has to be considered. The deformation measurement can also be expressed as a relative value related to the measurement at the initial time of measurement. The subsequent measurements can then be expressed as a percentage of the initial value. In case of uneventful healing, the measurement signal should demonstrate a continuous decrease over time until the fracture is united. 3 Finally, the load on the fracture can be induced as torsion between the proximal and distal part of the fracture. This method may provide the advantages of maintaining the bone axis and minimizing the risk of bone misalignment during the bone healing process but requires a dedicated measurement device. 4
An alternative way to directly assess the progress of fracture healing is the determination of the load share between the implant and the healing fracture (Fig. 11.2). The implant and the healing fracture constitute a composite structure sharing the loads between each other. The determination of the load share ratio between the implant and the healing bone requires the load through the implant to be measured while the composite structure is loaded. This can be performed by inserting load cells in the path of load transmission of the implant. Realistically, this can only be performed in the case of external fixation implants in which some of the rods can be temporarily replaced by load cells. Measuring the load share directly enables access to the amount of load that can be supported by the healing fracture and provides direct access to the load-bearing capacity of the fracture. However, the method is extremely complex, time-consuming, and not without risk for the patient as the fixation device has to be temporarily removed. 5
A general limitation of direct measurement of fracture stiffness is the exact knowledge of the load at the fracture site. If external loads are applied (by weight bearing), internal loads are generated by the muscles that superimpose the external loads and act at the site of fracture. Because it is almost impossible to control muscle activity of the patient during the measurement, the forces acting at the level of the fracture are not exactly known and consequently are a source of inaccuracy of the measurement. Another important source of measurement inaccuracy is loosening of fixator pins, which are either used to measure the deformation of the bone or to transfer load to the fixator body.3
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