Lang Yang

A biomechanical study of median sternotomy closure techniques.

OBJECTIVEnSternal dehiscence is a complication of median sternotomy incisions with high mortality and morbidity. Different techniques of sternal closure have been described. Rigid fixation of the sternum results in earlier union. We measured the rigidity of sternotomy fixation using a mechanical model in order to differentiate different techniques of sternal closure using their biomechanical characteristics.nnnMETHODSnWe measured the force-displacement curves of six different fixation techniques using a metal sternal model using a computerized materials-testing machine. We tested straight wires (the most commonly used surgical technique), figure-of-8 wires, repair technique (used when a wire breaks), Ethibond, Sterna-band and a multitwist closure described for the first time.nnnRESULTSnAt 20 kg force, twisted wires used for sternotomy closures start to untwist. The most rigid closure was a multitwist closure that displaced only 0.37 mm at a force of 20 kg. Straight wires displaced 0.78 mm, figure-of-8 wires 1.20 mm, Sterna-band 1.37 mm, repair wires 5.08 mm, Ethibond 9.37 mm. The single factor Anova test for the rigidity of the different closures had P-values <0.0001.nnnCONCLUSIONSnWe applied a mathematical model to calculate chest wall forces during coughing, in order to determine the force placed upon a sternotomy closure. We conclude that severe coughing may cause wires to untwist. We discuss potential applications of different wire closures based on their characteristics.

Fatigue testing median sternotomy closures

OBJECTIVEnSternal dehiscence is commonly due to wire cutting through bone. With a biological model, we measured the rate of cutting through bone, of standard steel wire closure, peristernal steel wire, figure-of-eight closure, polyester and sternal bands sternotomy closure techniques.nnnMETHODSnPolyester, figure-of-eight, peristernal and sternal band closures were tested against standard closure eight times using adjacent paired samples, to eliminate biological variables. Fatigue testing was performed by a computerized materials-testing machine, cycling between loads of 1 and 10 kg. The displacements at maximum and minimum loads were measured during each cycle. Cutting through, manifested by the displacement at the maximum load between the 1st and 150th cycles was measured. The percentage cut-through of each closure method versus standard closure was calculated.nnnRESULTSnThe differences in the displacement between each of the polyester (1.01 mm), figure-of-eight (0.52 mm), peristernal (0.72 mm) and sternal band (0.66 mm) groups versus standard closure (0.22, 0.22, 2.1, 3.2 mm) in the paired samples were statistically significant (Students paired t-test; P<0.01). There were statistically significant differences in the percentage cut-through of polyester, figure-of-eight, peristernal and sternal bands (ANOVA, P<0.001), versus standard closure.nnnCONCLUSIONSnIn our sheep sternum model, we have quantified the differing rate of cutting through bone of five types of median sternotomy closure techniques. We have controlled for bone variables by testing each closure versus standard closure using paired adjacent bone samples. Peristernal and sternal band closure techniques are significantly superior to standard closure. The use of polyester and figure-of-eight closures requires caution.

Bicondylar tibial plateau fractures managed with the Sheffield Hybrid Fixator: Biomechanical study and operative technique

The two main challenges in the management of bicondylar tibial plateau fractures are: Firstly, the compromised skin and soft tissue envelope which invite a high rate of complications following attempted open reduction and dual plating. Secondly, poor bone quality and comminuted fracture patterns, which create difficulty in achieving stable fixation. Although dual plating is considered to be the best mechanical method of stabilizing these complex fractures, there remains concern regarding the high rate of complications associated with extensive soft tissue dissection, required for the insertion of these plates in an already compromised knee. The Sheffield Hybrid fixator (SHF) technique offers a solution to the two main problems of these difficult fractures by minimizing soft tissue dissection, since bone fragments are reduced and fixed percutaneously, and providing superior cancellous bone purchase with beam loading stabilization for comminuted fractures. Our biomechanical testing showed the SHF with four tensioned wires to be as strong as dual plating and able to provide adequate mechanical stability in the fixation of bicondylar tibial plateau fractures. This was confirmed clinically by a prospective review of the use of the SHF at our centre, for managing complex and high-energy tibial plateau fractures with a good final outcome and no cases of deep infection or septic arthritis.

Limb reconstruction after high energy trauma

Limb reconstruction techniques rely on stable external fixation to provide early limb function after major long bone injury. Bone may be generated by callus distraction techniques and internal techniques of moving bone segments used to fill bone defects. Soft tissue defects may be treated by acute shortening, although skin defects will also close spontaneously during bone transport as the leading edge of bone is covered with granulation tissue. External fixation is also used to cross joints permitting rest and repair of the joint. Hinges placed within the bars of the fixation frame may be used to correct deformities in the bone and soft tissue contractures using closed distraction techniques. These techniques are appropriate to metaphyseal fractures and diaphyseal fractures with bone loss. A major advantage is the lack of donor site morbidity, associated with skin flaps and large bone grafts. Acceptance of these techniques is growing whilst the methodology continues to improve. In more complicated cases, specialist training and dedicated hospital units with multidisciplinary support is desirable.

The strength of different fixation techniques for bicondylar tibial plateau fractures--a biomechanical study

OBJECTIVEnTo compare the mechanical strength of fixation of bicondylar tibial plateau fractures using internal and external fixation techniques.nnnDESIGNnA randomised laboratory testing of fixation methods currently in clinical use.nnnBACKGROUNDnTo reduce the high complication rate associated with the double plating, many surgeons have started to use less invasive techniques. However the literature comparing the mechanical performance of these methods is still scarce.nnnMETHODSnBicondylar tibial plateau fractures were simulated on Sawbones tibiae and fixed with one of the five fixation methods: dual plating, a two-ring hybrid fixator with inter-fragmentary screws, a ring-bar hybrid fixator with inter-fragmentary screws, lateral plate and medial monolateral external fixator, lateral plate and medial inter-fragmentary screws. Following statistical power calculations, seven tibiae were used for each fixation method. The specimens were tested in cyclical compression to failure. The vertical subsidence in the medial or lateral plateau was measured using extensometers.nnnRESULTSnIn all cases the mode of failure was consistent with collapse occurring in the medial plateau. There was no significant difference in the failure load between dual plating and the two-ring hybrid fixator (4218 N and 4184 N respectively; P=0.28, t-test). Failure was seen at significantly lower loads with the other fixation systems.nnnCONCLUSIONnWeight-bearing mobilisation of the patient may be undertaken earlier with more confidence by using the double plating or two-ring hybrid fixator rather than other less strong techniques. The choice on which of these two methods to use may depend on tissue viability and surgeon preference.

The Sheffield hybrid fixator--a clinical and biomechanical review.

The Sheffield hybrid fixator (SHF) was developed to complement and extend the use of current monolateral fixation systems. Unlike other hybrid designs, it relies on rings for the diaphyseal as well as the metaphyseal support. Since it is an all ring system it may be used to cross joints and perform progressive deformity and contracture correction. Unlike the Ilizarov system only one ring is required at each level and diaphyseal transfixation is avoided. Initial biomechanical testing has demonstrated similar mechanical characteristics to an all wire Ilizarov fixator, however, increased shear motion was noted due to the stiffness asymmetry between the segments controlled by wires and screws. The first 100 cases were reviewed with good patient compliance and satisfactory results. Minor component problems were identified and corrected during this period and no significant complications occurred. The device is recommended for a wide range of complex trauma and limb reconstruction uses.

Techniques for improving stability in oblique fractures treated by circular fixation with particular reference to the sagittal plane

Biomechanical studies involving all-wire and hybrid types of circular frame have shown that oblique tibial fractures remain unstable when they are loaded. We have assessed a range of techniques for enhancing the fixation of these fractures. Eight models were constructed using Sawbones tibiae and standard Sheffield ring fixators, to which six additional fixation techniques were applied sequentially. The major component of displacement was shear along the obliquity of the fracture. This was the most sensitive to any change in the method of fixation. All additional fixation systems were found to reduce shear movement significantly, the most effective being push-pull wires and arched wires with a three-hole bend. Less effective systems included an additional half pin and arched wires with a shallower arc. Angled pins were more effective at reducing shear than transverse pins. The choice of additional fixation should be made after consideration of both the amount of stability required and the practicalities of applying the method to a particular fracture.

High-spatial-resolution bone densitometry with dual-energy X-ray absorptiometric region-free analysis.

PURPOSEnTo outline the conceptual development of dual-energy absorptiometric (DXA) region-free analysis, quantify its precision, and evaluate its application to quantify the change in longitudinal femoral periprosthetic bone mineral density (BMD) in patients during the 12 months after total hip arthroplasty.nnnMATERIALS AND METHODSnAll subjects had undergone total hip arthroplasty for idiopathic arthritis, and the scans were collected as part of previous ethically approved studies (1998-2005) for which informed consent was provided. Contemporary image processing approaches were used to develop a region of interest-free DXA analysis method with increased spatial resolution for assessment of proximal femoral BMD. The method was calibrated, and its accuracy relative to a proprietary algorithm was assessed by using a hip phantom. The precision of the method was examined by using repeat DXA acquisitions in 29 patients, and its ability to allow spatial resolution of localized periprosthetic BMD change at the hip was assessed in an independent group of 19 patients who were measured throughout a 12-month period. Differences were evaluated with t tests (P < .05).nnnRESULTSnThe method allowed spatial resolution of more than 10 000 individual BMD data points on a typical archived prosthetic hip scan. The median data point-level error of the method after calibration was -1.9% (interquartile range, -7.2% to 3.5%) relative to a proprietary algorithm. The median data point-level precision, expressed as a coefficient of variation, was 1.4% (interquartile range, 1.2%-1.6%). Evaluation of BMD change in a model of periprosthetic bone loss demonstrated large but highly focal changes in BMD that would not be resolved by using traditional region of interest-based analysis approaches.nnnCONCLUSIONnThe proposed approach provides a quantitative, precise method for extracting high-spatial-resolution BMD data from existing DXA datasets without the limitations imposed by region of interest-based analysis.

Use of high resolution dual-energy x-ray absorptiometry-region free analysis (DXA-RFA) to detect local periprosthetic bone remodeling events

Dual‐energy x‐ray absorptiometry (DXA) is the gold standard method for measuring periprosthetic bone remodeling, but relies on a region of interest (ROI) analysis approach. While this addresses issues of anatomic variability, it is insensitive to bone remodeling events at the sub‐ROI level. We have validated a high‐spatial resolution tool, termed DXA‐region free analysis (DXA‐RFA) that uses advanced image processing approaches to allow quantitation of bone mineral density (BMD) at the individual pixel (data‐point) level. Here we compared the resolution of bone remodeling measurements made around a stemless femoral prosthesis in 18 subjects over 24 months using ROI‐based analysis versus that made using DXA‐RFA. Using the ROI approach the regional pattern of BMD change varied by region, with greatest loss in ROI5 (20%, pu2009<u20090.001), and largest gain in ROI4 (6%, pu2009<u20090.05). Analysis using DXA‐RFA showed a focal zone of increased BMD localized to the prosthesis–bone interface (30–40%, pu2009<u20090.001) that was not resolved using conventional DXA analysis. The 20% bone loss observed in ROI5 with conventional DXA was resolved to a focal area adjacent to the cut surface of the infero‐medial femoral neck (up to 40%, pu2009<u20090.0001). DXA‐RFA enables high resolution analysis of DXA datasets without the limitations incurred using ROI‐based approaches.

The effect of tibial diaphyseal lengthening on the longitudinal growth of the tibia

Limb lengthening by tibial callotasis is usually performed in the metaphysis but may cause growth inhibition. Is diaphyseal lengthening more advantageous? Sixteen immature rabbits underwent 30% diaphyseal lengthening by tibial callotasis. The tibial length was measured on radiographs at the end of the distraction period and after an additional 5 weeks. The proximal and distal growth plates were assessed histomorphometrically. Osteotomy stimulated tibial elongation; however, combined with diaphyseal lengthening the stimulation was suppressed resulting in longitudinal growth that matched the control side. In longer lengthenings of limbs diaphyseal callotasis may be more advantageous than metaphyseal by not inhibiting longitudinal growth.