As further improvements in surgical accuracy are made possible by computer-aided surgery, there is a demand for new pre- and post- surgical assessment and more accurate intra-operative registration techniques. Ultrasonic palpation is being used in navigated hip surgery but as yet little work had been published on the identification of anatomical landmarks used in knee surgery with this technique. The aim of this study was to investigate the accuracy of the identification of the femoral condyles with ultrasound in both saline and in tissue mimicking material (TMM).

The system comprised of an image free navigation system (OrthoPilot, B Braun Aesculap) synchronized with a standard B-mode ultrasound system (Echoblaster 128, TELEMED) used with passive trackers. Bony anatomy was represented by two sawbone phantoms; one involving an isolated femur and one simulated knee joint. Both phantoms had fiducial markers in the form of steel pins inserted into the condylar eminences of the femur, providing sharply defined structural interfaces for determination of inter-condylar distance (ICD). Initial testing was completed in a waterbath filled with saline (NaCl 4500ppm) maintained at 22°C. Further testing used both sawbone phantoms encased in TMM. To gain accurate dimensions of the ICD, 3D models of both sawbone phantoms were created using a high-resolution non-contact 3D digitiser (Konica Minolta Sensing Inc.) and measurements taken using Geomagic software. Measurements for all test set-ups were repeated and mean (SD) values calculated.

The mean ICD measurement (SD) of the isolated femur from the high resolution 3D model was 53.6mm (1.2mm) (n=4). The ICD for the isolated femur in the saline water bath was 48.8mm (0.7mm) (n=5). For the isolated femur encased in TMM the mean ICD was 54.6mm (0.7mm) (n=4) with the probe positioned parallel to the shaft of the femur and 52.2mm (0.4mm) (n=5) with the probe held perpendicular to the femur. For the second phantom, which consisted of an articulated knee joint, the mean ICD measured from the high-resolution 3D model was 43.5mm (1.0mm) (n=5). When encased in TMM, the mean ICD derived from the navigation system was 42.6mm (1.4mm) (n=5).

Average ICD measurements for phantoms encased in TMM were within 1mm of that determined by high resolution, non-contact 3D digitization. However, results in the saline waterbath were less accurate, with an average difference of 4.8mm in ICD measurement. We believe these differences largely reflect the digitisation error associated with manual registration of the fiducial markers and highlights the difficulty in using this method and taking measurements within one scanned plane. Hence we are now developing a new method of automatic registration that uses multiple scans and will hopefully provide a more accurate outcome.

As well as improved component alignment, recent publications have shown that navigation systems can assess knee kinematics and provide a quantitative measurement of soft tissue characteristics. In particular, navigation-based measures of varus and valgus stress angles have been used to define of the extent of soft-tissue release required at the time of the placement of the prosthesis. However, the extent to which such navigation-derived stress angles reflect the restraining properties of the collateral ligaments of the knee remain unknown. The aim of this cadaveric study was to investigate correlations between the structural properties of the collateral ligaments of the knee and stress angles measured with an optically-based navigation system.

Nine fresh-frozen cadaveric knees (age 81 ± 11 years) were resected 10-cm proximal and distal to the knee joint and dissected to leave the menisci, cruciate ligaments, posterior joint capsule and collateral ligaments. The resected femoral and tibial were rigidly secured within a test system which replicated the lower limb and permitted kinematic registration of the knee using the standard workflow of a commercially available image free navigation system. Frontal plane knee alignment and varus-valgus stress angles in extension were acquired. The manual force required to produce varus-valgus stress angles during clinical testing was quantified with a dynamometer attached to the distal tibial segment. Following assessment of knee laxity, bone–ligament–bone specimens were prepared and mounted within a uniaxial materials testing machine. Following 10 preconditioning cycles specimens were extended to failure. Force and crosshead displacement were used to calculate principal structural properties of the ligaments including ultimate tensile strength and stiffness as well as the instantaneous stiffness at loads corresponding to those applied during varus-valgus stress testing. Differences in the structural properties of the collateral ligaments and the varus and valgus laxity of the knee were evaluated using paired t tests, while potential relationships were investigated with scatter plots and Pearson’s product moment correlations.

There was no significant difference in the mean varus (4.3 ± 0.6°) and valgus laxity measured (4.3 ± 2.1°) for the nine knees or the corresponding distal force application required during stress testing (9.9 ± 2.5N and 11.1 ± 4.2N, respectively). Six of the nine knees had a larger varus stress angle compared to the valgus angle. There was no significant difference in the stiffness of the medial (63 ± 15 N/mm) and lateral (57 ± 13 N/mm) collateral ligaments during failure testing. The medial ligament, however, was approximately two fold stronger than its lateral counterpart (780 ± 214N verse 376 ± 104N, p< 0.001). While the laxity measures of the knee were independent of the ultimate tensile strength and stiffness of the collateral ligaments, there was a significant correlation between the force applied during stress testing and the instantaneous stiffness of the medial (r = 0.91, p = 0.001) and lateral (r = 0.68, p = 0.04) collateral ligaments.

The findings of the current study suggest that computer-assisted measures of passive knee laxity are largely independent of the ultimate strength and stiffness of the collateral ligaments. The force applied during manual stress testing of the knee, however, was strongly correlated with the instantaneous stiffness of the collateral ligaments suggesting users may attend to the low-stress behaviour of the ligaments. Nonetheless the force applied during stress testing varied between knees, as did the resultant angular deviation. Therefore to make use of the quantified data given by navigation systems, further work to understand the relationships between applied force, resultant stress angles and clinical outcomes for knee arthroplasty is required.

Introduction: The geometry of uncemented press-fit ace-tabular cups is important in achieving primary stability to ensure bony ingrowth. This study compares the in vitro primary stability of two widely used designs.

Methods: The primary stability of two uncemented ace-tabular cup designs (true hemispheric and peripherally enhanced) with the same 52mm diameter and produced by the same manufacturer, was tested in vitro. Polyethylene blocks of low and high density -representing softer and harder bone- were reamed using the manufacturers’ reamers. The cups were seated using an Instron 5800R machine. Peak failure loads and moments during uniaxial pull-out and tangential lever-out tests were used as measures of primary stability. Eighty tests were performed.

Results: Low density substrate: no difference between the two designs for seating force or stability, with the substrate under-reamed by 2mm.

High density substrate: the cups could not be adequately seated with a 2mm under-ream. Seating was achieved with 1mm under-ream for the hemispheric and 1mm over-ream for the peripherally enhanced design. There was a statistically significant difference in seating forces, with the hemispheric cup requiring less force (6264±1535N vs 7858±2383N, p< 0.05). There was a statistically significant difference in the stability ratio of pull-out force to seating force, favouring the hemispheric cup.

Discussion: No difference was seen in the low density substrate between the 2 cups.

In the high density, the hemispheric design had better characteristics (lower seating force and higher pull-out force to seating force ratio) than the peripherally enhanced design, which are more favourable in clinical settings.