Medium- and long-term results of high tibial osteotomy using Garches external fixator and gait analysis for dynamic correction in varus osteoarthritis of the knee.

In arthritis of the varus knee, a high tibial osteotomy (HTO) redistributes load from the diseased medial compartment to the unaffected lateral compartment. We report the outcome of 36 patients (33 men and three women) with 42 varus, arthritic knees who underwent HTO and dynamic correction using a Garches external fixator until they felt that normal alignment had been restored. The mean age of the patients was 54.11 years (34 to 68). Normal alignment was achieved at a mean 5.5 weeks (3 to 10) post-operatively. Radiographs, gait analysis and visual analogue scores for pain were measured pre- and post-operatively, at one year and at medium-term follow-up (mean six years; 2 to 10). Failure was defined as conversion to knee arthroplasty. Pre-operative gait analysis divided the 42 knees into two equal groups with high (17 patients) or low (19 patients) adductor moments. After correction, a statistically significant (p < 0.001, t-test,) change in adductor moment was achieved and maintained in both groups, with a rate of failure of three knees (7.1%), and 89% (95% confidence interval (CI) 84.9 to 94.7) survivorship at medium-term follow-up. At final follow-up, after a mean of 15.9 years (12 to 20), there was a survivorship of 59% (95% CI 59.6 to 68.9) irrespective of adductor moment group, with a mean time to conversion to knee arthroplasty of 9.5 years (3 to 18; 95% confidence interval ± 2.5). HTO remains a useful option in the medium-term for the treatment of medial compartment osteoarthritis of the knee but does not last in the long-term. Cite this article: Bone Joint J 2016;98-B:601-7.

The Effects of Laser Marking and Symbol Etching on the Fatigue Life of Medical Devices.

This paper examines the question;" does permanent laser marking affect the mechanical performance of a metallic medical component?" The literature review revealed the surprising fact that very little has been presented or studied even though intuition suggests that its effect could be detrimental to a component's fatigue life. A brief investigation of laser marking suggests that defects greater than 25 µm are possible. A theoretical investigation further suggests that this is unlikely to cause issues with relation to fast fracture but is highly likely to cause fatigue life issues. An experimental investigation confirmed that laser marking reduced the fatigue life of a component. This combination of lines of evidence suggests, strongly, that positioning of laser marking is highly critical and should not be left to chance. It is further suggested that medical device designers, especially those related to orthopaedic implants, should consider the position of laser marking in the design process. They should ensure that it is in an area of low stress amplitude. They should also ensure that they investigate worst-case scenarios when considering the stress environment; this, however, may not be straightforward.

Measuring multi-dimensional, time-dependent mechanical properties of a human tibial fracture using an automated system.

This paper presents an element of a long-term research project determining a clinically quantifiable end point for fracture healing in humans. An automated loading and measurement device is presented. It has been developed as a research tool for the assessment of the mechanical properties of a healing human tibial fracture. The device has been specifically designed for use with patients treated with external fixation. The characteristics of the device have been presented together with an assessment of errors. A typical sample of results has been presented to demonstrate the significance of the device; subsequent papers will examine the whole data set in greater depth. The results presented here confirm the non-linear behaviour of callus and reinforce the requirement to measure the load rate when measuring the fracture stiffness. A new material property for the assessment of fracture healing, namely gamma, is examined and preliminary results are shown. Polar plots of stiffness demonstrate that, when measuring fracture stiffness, not only should the load rate be considered, but also the orientation of measurement. The results from this work support the view that the fracture stiffness should be measured in at least two planes. Currently a fracture can be considered healed when the fracture stiffness exceeds 15 N m/deg; this paper questions whether this value is now valid and suggests that it should be re-examined.

Mechanical properties of callus in human tibial fractures: a preliminary investigation.

OBJECTIVE: Investigate the mechanical properties of callus in tibial fractures treated with external fixation. In particular any time-dependent or non-linear characteristics. DESIGN: A clinical-based investigative study. BACKGROUND: A fracture bending stiffness in the sagittal plane of 15 N m/deg. has been stated as a satisfactory endpoint of clinical healing. However using this criterion in 76 previous tibial fractures, four continued to a malunion. METHODS: Fracture properties were measured in clinic. The fixator was removed for the tests and a specially designed system was used to measure displacement and load. Fracture stiffness was measured in different planes and at various loading rates. Passive stressing of the leg was performed whilst fracture displacement was recorded. A constant load was applied for a longer period to assess creep properties. RESULTS: Fracture stiffness was found to vary between different planes of measurement and on load rate. The visco-elastic characteristics of the callus changed with time. In early measurements, the callus absorbed a large proportion of energy when a load was applied. Later tests showed a progressive change with the callus absorbing less energy. This demonstrates that the properties of the callus changed with time, with the viscous element diminishing and the elastic element increasing. This sometimes occurred with no change in the measured fracture stiffness. CONCLUSIONS: Further investigation is needed, focusing on the visco-elastic properties of callus, to develop a more reliable method of determining clinical union. RELEVANCE: Improved understanding of the mechanical properties of callus could result in improved methods of measuring fracture healing.

Cortex distraction forces caused by the insertion of external fixator pins.

OBJECTIVE: Insertion of a bicortical threaded external fixator pin can cause bone damage. An unexpected crack can propagate along the bone when the pin touches the far cortex. The objective of this study was to investigate whether drilling and inserting bicortical pins into the shafts of long bones can cause large distraction forces to be generated between the cortices. DESIGN: Two flat samples of bone or bone substitute (Tufnol) were mounted parallel at each end of a specially designed force measurement column. Three common pin designs were inserted into the samples: tapered, self-threading and self-drilling, and self-threading pins. The axial thrust and the cortex distraction force between the two samples were measured as the pins were inserted. RESULTS: High distraction forces were measured between the cortices for all pin designs, typically 700 to 1,000 Newtons. CONCLUSIONS: The clinical significance of these findings is that the insertion of bicortical pins of the designs tested may generate a large distraction force between the cortices. This may drive a fracture apart or propagate an undisplaced fracture. Therefore, care must be taken to ensure that bicortical pins are inserted in locations free of local defects.

A fracture movement monitoring system to aid in the assessment of fracture healing in humans.

This paper presents a new design for a device to monitor the motion of fracture fragments in diaphyseal tibial fractures. The device measures the motion that occurs at the fracture site when loaded by gait or by manipulation. It has undergone rigorous calibration and acceptance trials. The device has been used in ethically approved research clinics held at the North Staffordshire Hospital (40 patients). The paper presents a selection of results obtained using the new device. The results demonstrate several new ways of assessing fracture healing by examining fracture site motion. The following conclusions were drawn: 1. If fracture monitoring devices are to be attached to bone screws, it is essential to minimize bone screw errors. To do this, each patient must have similar bone screw lengths, orientations, alignment and siting. This is only achievable using a peroperative reduction device. 2. If fracture stiffness is to be used as a measure of fracture healing, load rate should be controlled; at the very least strain rate should be controlled. 3. It is imperative that fracture stiffness be measured in more than one plane by a biplanar device so that asymmetry may be accommodated. 4. Fracture stiffness, on its own, is probably not a sufficiently rigorous measure of healing end-point. The quantifiably viscoelastic properties of healing callus should be taken into account.

A device for improved reduction of tibial fractures treated with external fixation.

A widely used method of treatment for unstable tibial shaft fractures is unilateral external fixation. The majority of fixators act as three distinct devices: an intra-operative reduction device, a device to maintain fracture alignment during healing and an aid to healing by allowing movement at the fracture site. Conventional operative techniques require the surgeon to manipulate a number of degrees of freedom at once, making reduction of the fracture difficult, and results in the fixator being out of alignment with the long axis of the bone. An operative method has been developed that separates reduction and fixation. A dedicated device has been designed to improve the per-operative control of fracture fragments during fracture reduction. The device has been used in clinical trials for the reduction of 22 diaphyseal tibial fractures. Compared with previous operative techniques there has been a saving of 53 per cent in fracture reduction time and an overall saving of 10 per cent in operating time. Fracture alignment has been improved compared with reductions achieved with a fixator which potentially improves healing and lowers the rate of malunion. In each case the fixator has been applied in alignment with the bone, improving dynamization and reducing the likelihood of malunion due to fixator cam slippage.

A data-logging system to monitor bone fracture movement continuously.

A compact system is presented, which can continuously monitor the occurrence of fracture site movement in patients with tibial shaft fractures treated with an Orthofix external fixator allowing limited axial movement. The system comprises a microswitch and data logger board, which are both attached to the fixator. Each time the switch is closed by the movement of the fixator, the data logger records it as an event. The number of events per half hour is stored to the board's memory. The data logger can record the frequency of movement for at least a four-week period, at which time, the data can be transferred to a computer via a serial link. The system has been proven in a limited patient trial. The results highlight the variation in patient mobility. The significance of continuously monitoring the fracture site movement during the healing period is discussed.