A portable monitoring system for measuring weight-bearing during tibial fracture healing

doi:10.1016/S1350-4533(98)00061-7

Medical Engineering & Physics

Volume 20, Issue 7, October 1998, Pages 543-548

https://doi.org/10.1016/S1350-4533(98)00061-7 Get rights and content

Abstract

An ambulatory monitoring system has been developed to measure the weight-bearing achieved by a patient on their fractured leg. The system is lightweight, portable, and can monitor weight-bearing continuously over an extended period of time. In addition to mean weight-bearing, the system can also determine temporal parameters associated with the patients' gait. This system was used to measure the weight-bearing achieved during the course of fracture healing in a small number of patients being treated by a variety of fixation methods for a tibial fracture. In general, weight-bearing was observed to increase with time post fracture, and the fixation method employed appeared to influence the rate of increase of loading of the fractured limb, more rigid methods of fixation enabling full-weight-bearing to be achieved sooner.

Introduction

Fracture healing is influenced by the mechanical environment prevailing at the fracture site 1, 2, 3, 4, 5. Fractures which have been accurately reduced and rigidly fixed, and in which there is minimal interfragmentary movement, heal by so called direct or primary healing 1, 2. Fractures which have been less rigidly fixed heal with the production of an external callus (this healing being termed `indirect healing'). The amount of callus produced depends on the rigidity of fixation, less rigidly fixed fractures producing more callus 1, 3, 4. For fractures treated with the same type of fixation, the daily application of a small (⩽1 mm) amount of cyclic micromovement has been shown to induce an earlier formation of periosteal callus compared with unstimulated controls 4, 5.

The rate of fracture healing, in terms of the increase of fracture stiffness and strength, can be influenced by the rigidity of the fixation system and the mechanical conditions pertaining at the fracture site, this being seen in both experimental 3, 4, 5and clinical studies 6, 7. To encourage fracture healing therefore, early weight-bearing is prescribed and encouraged to generate the strains at the fracture site necessary to promote callus formation [7]. Weight-bearing can generate significant amounts of movement at the fracture site. The largest axial movements (1–4 mm) are seen in patients treated by cast 8, 9, less movement (1–2 mm) being observed in patients treated with more rigid methods of fixation such as external skeletal fixation 10, 11, 12, 13.

The magnitude and frequency of weight-bearing will therefore determine how a fracture is loaded, how much movement (and therefore strain) is produced at the fracture site and thus should be an important influence on healing. Information on weight-bearing during healing would be invaluable is assessing the relative effects of treatment method, injury grade and patient motivation on the ability of the patient to weight-bear on the fractured leg. In previous studies of patients being treated by cast braces for femoral shaft fractures, the loading of the fractured limb during healing has been measured 14, 15and has been shown to increase with time post fracture. In patients treated with external skeletal fixation for a tibial fracture, weight bearing was found to be less than 50% of body weight during the first two months post fracture [10]. In all of these previous studies however, weight bearing was assessed from measurements made when the patient walked over a force plate, and direct inference cannot therefore be made that such weight bearing was the norm during normal patient activity.

In this study, a small portable monitoring system was developed to measure the load which a patient puts through a tibial fracture. This system was subsequently used to measure the weight-bearing achieved with time in patients being treated by a variety of fixation methods for a tibial fracture.

Section snippets

Materials and methods

The monitoring system was developed on a commercially available microprocessor system (Mini Module, PSI Systems, Essex, U.K.). Included in this system are a central processing unit (CPU) consisting of a Philips 93C100 microprocessor, erasable programmable read only memory (EPROM), random access memory (RAM) with lithium battery back-up, a real time clock, 16 digital channels, four analogue to digital converters, one digital to analogue converter, an RS485 serial interface, a keyboard adapter,

Results

During these preliminary trials, a total of 37 patients with tibial fractures were monitored at least once during the course of their treatment. Of these, nine patients had three or more consecutive measurements performed. From these, the results for three patients, two treated with an intramedullary nail and one treated with an external fixator, are given below, these having had the greatest number of measurement sessions during fracture healing. Due to the small number of consecutive

Discussion

The monitoring system developed was able to measure weight-bearing continuously during normal patient ambulation. This is an improvement over previous studies in which a single measurement or a small series of measurements of weight-bearing were obtained, often in an unfamiliar environment. As the weight-bearing data is averaged over the whole duration of the walk, it may therefore be more representative of the loading (and therefore stimulation) which the patient will give to the fracture

Conclusions

A portable system has been developed which records the magnitude and duration of weight-bearing during ambulation. This system was used to monitor changes in weight-bearing and the timing of weight-bearing in the fractured leg of a small series of patients with a tibial fracture.

Acknowledgements

This work was supported by a grant from SPARKS.

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During the first 4 weeks after surgery, the patient is not allowed to apply body weight and advised to walk with the help of a pair of crutches [21,23]. According to the experimental study of Aranzulla et al. [25], at 6 weeks post fracture, the weight bearing load of the fractured bone was reported to be ~50% of the body weight. Gardner et al. [6] show that the callus develops drastically between 4 and 8 weeks after surgery.
The evolutionary healing phenomenon of fractured tibia bone was investigated by comparing the bio-mechanical response of the human tibia following fracture fixation for two ranges of patient ages, when a body weight pressure (BWP) is applied. Three-dimensional finite element models have been developed by adopting the biomechanical characteristics of cortical and trabecular tibia bones, and considering the time-varying callus properties during the healing process for the two patients. The stress and strain levels generated within the fractured tibia bone by the screw tight fit during the assembly process revealed its dependence on the bone stiffness that degrades with age. They have an impact on primary stability of the implants prior to the osseointegration. The gap capacity to resist and allow a gradual BWP load transfer, through the callus for the tibia bone models, was analyzed. In fact, from 10 weeks after surgery, the callus allowed the BWP transfer for young patients, which guarantees sufficient structure stabilization of the fractured tibia. However, an insufficient load was transferred to the fracture gap for the old patient, even beyond 16 weeks, which delayed the bone consolidation
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Until such technology is feasible for large scale clinical use decades of development are still needed. To recruit a large number of patients for trials and ultimately treatment a more universal surrogate parameter for healing is needed: Gait is increasingly viewed as a future surrogate parameter for fracture monitoring of the lower extremities and has shown good preliminary correlation with fracture healing [20,21]. Studies were able to show a clear correlation between the biomechanical fracture stiffness and weight bearing [22].
It is well established that local mechanical conditions and interfragmentary movement are important factors for successful bone healing and may vary dramatically with patient fracture-load and activity. Up until now however it was technically impossible to use these key influence parameters in the aftercare treatment process of human lower extremity fractures. We propose a theory that with state of the art sensor technology these biomechanical influences can not only be monitored in vivo, but also used for individualized therapy protocols.
Local measurement systems for fracture healing are available but remain research tools, due to various technical issues. To investigate the biomechanical influences on healing right away surrogate sensor tools are needed. Various gait characteristics have been proposed as surrogate measures. Currently available sensor tools could be modified with the appropriate support structure to allow such measurements continuously over the course of a fracture healing. Interdisciplinary work between clinicians, software engineers with computer and biomechanical simulations is needed.
Through such a sensor system human boundary conditions for fracture healing could not only be defined for the first time, but also used for a unique, extendible aftercare system. With this tool critical healing situations would be detected much earlier and could be prevented with easy activity modifications, reducing patient and socioeconomic burden of disease. The hypothesis, necessary tools and support structures are presented.
Gait and function as tools for the assessment of fracture repair - The role of movement analysis for the assessment of fracture healing
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Another way of measuring the stage of fracture healing is the assessment of gait patterns [20,23,24]. The ability of patients to tolerate loads in fractured limb is directly proportional to the consolidation stage and the inability to bear weight can cause changes in gait patterns, resulting in antalgic gait [20,23,24]. Ambulation shows up a basic necessity for performing various daily activities, and their analysis provides important information about the functional capacity of the individual [25,26].
Assessment of gait and function might be as sensitive tool to monitor the progress of fracture healing. Currently available assessment tools for function use instrumented three dimensional gait analysis or pedobarography. The analysis is focused on gait or movement parameters and seeks to identify abnormalities or asymmetries between legs or arms. The additional inclusion of muscle function by electromyography can further elucidate functional performance and its temporal development. Alternative approaches abstain from directly assessing function in the laboratory but rather determine the amount of activities of daily living or the mere ability to perform defined tasks such as walking, stair climbing or running. Some of these methods have been applied to determine recovery after orthopaedic interventions including fracture repair. The combination of lab-based functional measurements and assessment of physical activities in daily live may offer a valuable level of information about the gait quality and quantity of individual patients which sheds light on functional limitations or rehabilitation of gait and mobility after a disease or injury and the respective conservative, medical or surgical treatment.
Biomechanical methods for the assessment of fracture repair
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In these patients it might be safer to rely on the patients’ subjective self-assessment to start and increase weight bearing. It has been shown that unrestricted weight bearing increases with time after fracture [12,13]. Joslin et al. demonstrated that patients who recovered well reached weight bearing of 90% of normal, while patients with delayed union achieved only 40% of normal at same time [14].
The progress of fracture healing is directly related to an increasing stiffness and strength of the healing fracture. Similarly the weight bearing capacity of a bone directly relates to the mechanical stability of the fracture. Therefore, assessing the progress of fracture repair can be based on the measurement of the mechanical stability of the healing fracture. However, fracture stability is difficult to assess directly due to various obstacles of which shielding of the mechanical properties by the fracture fixation construct is the most relevant one. Several assessment methods have been proposed to overcome these obstacles and to obtain some sort of mechanical surrogate describing the stability of the fracture. The most direct method is the measurement of the flexibility of a fracture under a given external load, which comprises the challenge of accurately measuring the deformation of the bone. Alternative approaches include the measurement of load share between implant and bone by internal or by external sensors. A direct 3 dimensional measurement of bone displacement is provided by radiostereometric analysis which can assess fracture migration and can detect fracture movement under load. More indirect mechanical methods induce cyclic perturbations within the bone and measure the response as a function of healing time. At lower frequencies the perturbations are induced in the form of vibration and at higher frequencies in the form of ultrasonic waves. Both methods provide surrogates for the mechanical properties at the fracture site. Although biomechanical properties of a healing fracture provide a direct and clinically relevant measure for fracture healing, their application will in the near future be limited to clinical studies or research settings.
A novel intramedullary nail for micromotion stimulation of tibial fractures
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In another study, small interfragmentary displacements of approximately 0.2 mm in an external fixator of known spring rate can be correlated with a total stance-phase load of only 40 N (Kenwright and Goodship, 1989). Some authors have hypothesised that a biofeedback mechanism mediated by mecho-receptors restricts weight-bearing and prevents patients from over-loading the healing fracture (Joslin et al., 2008), a theory which is supported by observations that patients with very rigid fixators (i.e. IM nails) can bear more weight more quickly because the implant bears much of the load (Aranzulla et al., 1998). However, this has also been cited as a weakness of IM nails in that controlled callus compression does not occur and patients with full non-union may be able to fully bear weight on an IM nail (Joslin et al., 2008).
Animal studies and clinical trials have suggested that early application of controlled axial micromotion can accelerate healing of long bone fractures compared to rigid fixation. However, experimental investigations of micromotion constructs have been limited to external fixators, which have a higher incidence of complications than intramedullary nails. The purpose of this study was to assess whether a novel intramedullary nail design can generate stimulatory micromotion under minimal weight-bearing loads typical of the early healing period.
Eight cadaver tibiae were reamed, osteotomised, and implanted with commercially-available IM nails fitted with a custom insert that allowed 1 mm of axial micromotion after proximal/distal interlocking. Specimens were mounted in a materials testing machine and subjected to cyclic axial loading while interfragmentary motion was measured using an extensometer. Implants were also tested in standard statically-locked mode.
The average force required to cause distraction of the fracture gap in micromotion mode was 37.0 (SD 21.7) N. The mean construct stiffness was 1046.8 (SD 193.6) N/mm in static locking mode and 512.4 (SD 99.6) N/mm in micromotion mode (significantly different, P < 0.001).
These results support the development of a micromotion-enabled IM nail because the forces required to cause interfragmentary movements are very low, less than the weight of the hanging shank and foot. In contrast to rigid-fixation nails, which require significant weight-bearing to induce interfragmentary motion, the micromotion-enabled nail may allow movement in non-weight-bearing patients during the early healing period when the benefits of mechanical stimulation are most critical.

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CommunicationA portable monitoring system for measuring weight-bearing during tibial fracture healing

Abstract

Introduction

Section snippets

Materials and methods

Results

Discussion

Conclusions

Acknowledgements

J Biomed Eng

Clin Biomech

J Biomech

The biology of fracture healing in long bones

J Bone and Joint Surg

Physical and biological aspects of fracture healing with special reference to internal fixation

Clin Orthop Rel Res

Fracture healing in rat femora as affected by functional weight bearing

J Bone and Joint Surg

The influence of induced micromovement upon the healing of experimental tibial fractures

J Bone and Joint Surg

Communication
A portable monitoring system for measuring weight-bearing during tibial fracture healing