It is nowadays widely recognized that patient satisfaction following knee arthroplasty strongly depends on ligament balancing. To obtain this balancing, the occurring ligament strain is assumed to play a crucial role. To measure this strain, a method is described in this paper that allows full field 3D evaluation of the strains. The latter is preferred over traditional measurement techniques, e.g. displacement transducers or strain gauges, as human soft tissue is not expected to deform uniformly due to its highly inhomogeneous and anisotropic properties. To facilitate full field strain measurements, the 3D digital image correlation (DIC) technique was adopted. This technique was previously validated by our research group on human tissue. First, a high contrast speckle pattern was applied on the sMCL. Therefore, the specimens are first coated with a small layer of methylene blue. Following, a random white speckle pattern is applied. During knee flexion, two cameras simultaneously take pictures of the deforming region at predefined flexion angles. Using dedicated software, the captured images are eventually combined and result in 3D full field strains and displacements. Using this method, the strain distribution was studied in six cadaveric knees during flexion extension movement. Therefore, the femur was rigidly fixed in a custom test rig. The tibia was left unconstrained, allowing the six degrees of freedom in the knee. A load was applied to all major muscles in physiological directions of each muscle by attaching a series of calibrated weights (Farahmand et al., J Orthop Res., 1998;16(1)). The direction of the pulling cables was controlled using a digital inclinometer for each specimen. As a result, a statically balanced muscle loading of the knee was obtained. From these cadaveric experiments, it is observed that on average the sMCL behaves isometrically between 0° and 90° of flexion. However, high regional differences in strain distribution are observed from the full field measurements. The proximal region of the sMCL experiences relatively high strains upon flexion. These strains are positive (tension) in the anterior part and negative (compression) in the posterior region. In contrast, the distal region remains approximately isometric upon knee flexion (see Figure 1). It is accordingly concluded that the sMCL behaves isometric, though large regional differences are observed. The proximal region experiences higher strains. Furthermore, the DIC technique provided valuable insights in the deformation of the sMCL. This technique will therefore be applied to study the impact of knee arthroplasty in the near future. Caption with figure 1: Full field strain distribution in the sMCL's longitudinal direction for specimen in 45° (a) and 90° (b) of knee flexion
Anterior positioning of a cephomedullary nail in the distal femur occurs in up to 88% of cases. This is considered to occur because of a mismatch between the radius of curvature of the femur and that of available implants. The hypothesis for this study was that the relative thicknesses of the cortices of the femur (referenced off the linea aspera) change with age and determine the final position of intramedullary implants. This study used the data from CT scans undertaken as part of routine clinical practice in 919 patients with intact left femora (median age 66 years, 484 male and 435 female). The linea aspera and transverse intervals were plotted on a template femur between 25% – 60% femoral bone length (5% increments) and mapped automatically to all individual femora in the database with measurements taken in the plane of the linea aspera. The linea aspera was found to be internally rotated as compared to the sagittal plane referenced off the posterior femoral condyles. An age related change in the posterior/anterior cortical thickness ratio was demonstrated. The >80 year old cohort shows a significantly disproportional posterior/anterior ratio increase of 70.0% from 25–50% bone length as compared to 48.1% for the <40 year old cohort (p<0.05). This study has shown that assessment in the sagittal plane may be inaccurate because of rotational changes in the linea aspera. The centering influence of the corticies is lost with age with a relative thinning of the anterior cortex and thickening of the posterior cortex moving distally in the femur. This has a direct influence on the positioning of intramedullary implants explaining the preponderance of anterior malpositioning of intramedullary implants in the elderly.
The poor outcome of large head metal on metal total hip replacements (LHMOMTHR) in the absence of abnormal wear at the articulating surfaces has focussed attention on the trunnion/taper interface. The RedLux ultra-precision 3D form profiler provides a novel indirect optical method to detect small changes in form and surface finish of the head taper as well as a quantitative assessment of wear volume. This study aimed to assess and compare qualitatively the tapers from small diameter with LHMOMTHR's. 3 groups of retrieval tapers were analysed (Group 1: 28mm CoCr heads from MOMTHRs (n=5); Group 2: Large diameter CoCr heads from LHMOMTHRs (n=5); Group 3: 28mm heads from metal on polyethylene (MOP) THRs; n=3). Clinical data on the retrievals was collated. Both bearing surfaces and head tapers were measured for wear using the Redlux profiling non contact measurement system. Measurements included taper angle and 3D surface maps. Taper angles obtained with the Redlux method were compared to those obtained using CMM measurement on 3 parts. The Redlux profiling, including imprints, was also repeated 3 times to gauge potential errors. There was no difference in mean 12/14 taper angles between groups. There was no difference in volumetric and linear wear at the bearing surface between groups. Only the LHMOMs showed transfer of pattern from the stem to the internal head taper, with clear demarcation of the contact and damaged area between head taper and stem trunnion.
The poor outcome of large head metal on metal total hip replacements (LHMOMTHR) in the absence of abnormal wear at the articulating surfaces has focussed attention on the trunnion / taper interface. The RedLux ultra-precision 3D form profiler provides a novel indirect optical method to detect small changes in the form and surface finish of the head taper as well as a quantitative assessment of wear volume. This study aimed to assess and compare qualitatively the tapers from well functioning small diameter, with poorly functioning LHMOMTHR's using the above technique. Method. 3 groups of retrieval tapers were analysed (Group 1: 28 mm CoCr heads from well functioning MOMTHRs (n=5); Group 2: Large diameter CoCr heads from LHMOMTHRs revised for failure secondary to adverse reaction to metal debris (n=5); Gp 3 (control): 28 mm heads from well functioning metal on Polyethylene (MOP) THRs; n=3). Clinical data on the retrievals was collated. The Redlux profiling of modular head tapers involves a non direct method whereby an imprint of the inside surface of a modular head is taken, and this is subsequently scanned by an optical non contact sensor using dedicated equipment [1]. The wear was also measured on the bearing surface [1]. RedLux profiling of the tapers produced a taper angle and 3D surface maps. The taper angles obtained with the Redlux method were compared to those obtained using CMM measurement on 3 parts. The Redlux profiling, including imprints, was also repeated 3 times to gauge potential errors. Results. There was no difference in mean 12/14 taper angles between groups. There was no difference in volumetric and linear wear at the bearing surface between groups. Only the LHMOMs showed transfer of pattern from the stem to the internal head taper, with clear demarcation of the contact and damaged area between head taper and stem trunnion (see figure 1 – interpretation of head taper surface features demonstrated using Redlux optical imaging).
