Objectives

Articular cartilage damage is a primary outcome of pre-clinical and clinical studies evaluating meniscal and cartilage repair or replacement techniques. Recent studies have quantitatively characterized India Ink stained cartilage damage through light reflectance and the application of local or global thresholds. We develop a method for the quantitative characterisation of inked cartilage damage with improved generalisation capability, and compare its performance to the threshold-based baseline approach against gold standard labels.

As treatments of knee osteoarthrosis are continually refined, increasingly sophisticated methods of evaluating their biomechanical function are required. Whilst TKA shows good preoperative pain relief and survivorship, functional outcomes are sub-optimal, and research focus has shifted towards their improvement. Restoration of physiological function is a common design goal that relies on clear, detailed descriptions of native biomechanics. Historical simplifications of true biomechanisms, for example sagittal plane approximation of knee kinematics, are becoming progressively less suitable for evaluation of new technologies. The patellar tendon moment arm (PTMA) is an example of such a metric of knee function that usefully informs design of knee arthroplasty but is not fully understood, in part due to limitations in its measurement. This research optimized PTMA measurement and identified the influence of knee size and sex on its variation.

The PTMA about the instantaneous helical axis was calculated from optical tracked positional data. A fabricated knee model facilitated calculation optimization, comparing four data smoothing techniques (raw, Butterworth filtering, generalized cross-validated cubic spline-interpolation and combined filtering/interpolation). The PTMA was then measured for 24 fresh-frozen cadaveric knees, under physiologically based loading and extension rates. Sex differences in PTMA were assessed before and after size scaling.

Large errors were measured for raw and interpolated-only techniques in the mid-range of extension, whilst both raw and filtered-only methods saw large inaccuracies at terminal extension and flexion. Combined filtering/interpolation enabled sub-mm PTMA calculation accuracy throughout the range of knee flexion, including at terminal extension/flexion (root-mean-squared error 0.2mm, max error 0.5mm) (Figure 1).

Before scaling, mean PTMA throughout flexion was 46mm; mean, peak, and minimum PTMA values were larger in males, as was the PTMA at terminal flexion, the change in PTMA from terminal flexion to peak, and the change from peak to terminal extension (mean differences ranging from 5 to 10mm, p<0.05). Knee size was highly correlated with PTMA magnitude (r>0.8, p<0.001) (Figure 2). Scaling eliminated sex differences in PTMA magnitude, but peak PTMA occurred closer to terminal extension in females (female 15°, male 29°, p=0.01) (Figure 3).

Improved measurement of the PTMA reveals previously undocumented characteristics that may help to improve the functional outcomes of knee arthroplasty. Knee size accounted for two-thirds of the variation in PTMA magnitude, but not the flexion angle at which peak PTMA occurred, which has implications for morphotype-specific arthroplasty and musculoskeletal models. The developed calculation framework is applicable both in vivo and vitro for accurate PTMA measurement and might be used to evaluate the relative performance of emerging technologies.

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Combined Partial Knee Arthroplasty (CPKA) is a promising alternative to Total Knee Arthroplasty (TKA) for the treatment of multi-compartment arthrosis. Through the simultaneous or staged implantation of multiple Partial Knee Arthroplasties (PKAs), CPKA aims to restore near-normal function of the knee, through retention of the anterior cruciate ligament and native disease-free compartment. Whilst PKA is well established, CPKA is comparatively novel and associated biomechanics are less well understood.

Clinically, PKA and CPKA have been shown to better restore knee function compared to TKA, particularly during fast walking. The biomechanical explanation for this superiority remains unclear but may be due to better preservation of the extensor mechanism. This study sought to assess and compare extensor function after PKA, CPKA, and TKA.

An instrumented knee extension rig facilitated the measurement extension moment of twenty-four cadaveric knees, which were measured in the native state and then following a sequence of arthroplasty procedures. Eight knees underwent medial Unicompartmental Knee Arthroplasty (UKA-M), followed by patellofemoral arthroplasty (PFA) thereby converting to medial Bicompartmental Knee Arthroplasty (BCA-M). In the final round of testing the PKA implants were removed a posterior-cruciate retaining TKA was implanted. The second eight received lateral equivalents (UKA-L then BCA-L) then TKA. The final eight underwent simultaneous Bi-Unicondylar Arthroplasty (Bi-UKA) before TKA. Extensor efficiencies over extension ranges typical of daily tasks were also calculated and differences between arthroplasties were assessed using repeated measures analysis of variance.

For both the medial and lateral groups, UKA demonstrated the same extensor function as the native knee. BCA resulted in a small reduction in extensor moment between 70–90° flexion but, in the context of daily activity, extensor efficiency was largely unaffected and no significant reductions were found. TKA, however, resulted in significantly reduced extensor moments, leading to efficiency deficits ranging from 8% to 43% in flexion ranges associated with downhill walking and the stance phase of gait, respectively.

Comparing the arthroplasties: TKA was significantly less efficient than both UKA-M and BCA-M over ranges representing stair ascent and gait; TKA showed a significant 23% reduction compared to BCA-L in the same range. There were no differences in efficiency between the UKAs and BCAs over any flexion range and TKA efficiency was consistently lower than all other arthroplasties.

Bi-UKA generated the same extensor moment as native knee at flexion angles typical of fast gait (0–30°). Again, TKA displayed significantly reduced extensor moments towards full extension but returned to the normal range in deep flexion. Overall, TKA was significantly less efficient following TKA than Bi-UKA.

Recipients of PKA and CPKA have superior functional outcomes compared to TKA, particularly in relation to fast walking. This in vitro study found that both UKA and CPKA better preserve extensor function compared to TKA, especially when evaluated in the context of daily functional tasks. TKA reduced knee extensor efficiency by over 40% at flexion angles associated with gait, arguably the most important activity to maintain patient satisfaction. These findings go some way to explaining functional deficiencies of TKA compared to CPKA observed clinically.

Introduction

Pre-clinical assessment of total knee replacements (TKR) can provide useful information about the constraint provided by an implant, and therefore help the surgeon decide the most appropriate configurations. For example, increasing the posterior tibial slope is believed to delay impingement in deep flexion and thus increase the maximal flexion angle of the knee, however it is unclear what effect this has on anterior-posterior (AP) constraint.

The current ASTM standard (F1223) for determining constraint gives little guidance on important factors such as medial- lateral (M:L) loading distribution, flexion angle or coupled secondary motions. Therefore, the aim of the study was to assess the sensitivity of the ASTM standard to these variations, and investigate how increasing the posterior tibial slope affects TKR constraint.

Introduction

In total knee arthroplasty (TKA) the knee may be found to be too stiff in extension, causing a flexion contracture. One proposed surgical technique to correct this extension deficit is to recut the distal femur, but that may lead to excessively raising the joint line. Alternatively, full extension may be gained by stripping the posterior capsule from its femoral attachment, however if this release has an adverse impact on anterior-posterior (AP) stability of the implanted knee then it may be advisable to avoid this technique. The aim of the study was therefore to investigate the effect of posterior capsular release on AP stability in TKA, and compare this to the restraint from the cruciate ligaments and different TKA inserts.

Background

Medical advances and an ageing population mean that more people than ever rely on artificial joints. In the past years, shoulder joint replacement has developed rapidly and the numbers of shoulder prostheses implanted increased dramatically. Wear is one of the main contributors to the failure of shoulder implants. It is therefore important to measure the wear properties of the articulating surfaces within the joint in vitro. Investigation of wear characteristics through a comprehensive range of motion using a sophisticated shoulder simulator would reveal the durability of the material, the performance of component design and the safety analyses of prostheses. The purpose of the work was to develop and validate a multi-station shoulder simulator, which could accurately simulate physiological gleno-humeral forces and displacements during activities of daily living.

Introduction

There is little information available to surgeons regarding how the lateral soft-tissue structures prevent instability in knees implanted with total knee arthroplasty (TKA). The aim of this study was to quantify the lateral soft-tissue contributions to stability following cruciate retaining (CR) TKA.

Introduction

Hemiarthroplasty is an attractive technique for young and active patients as it preserves more bone stock. Polycarbonate urethane (PCU) has recently been introduced as an alternative bearing material. DSM Biomedical BV (Geleen, The Netherlands) has modified Bionate^® PCU 80A (80AI) with C18 groups and produce Bionate^® II PCU 80A (80AII) to create a different biointerface and enhance its tribological properties. The aim of this study was to compare friction performance of the articulation of the cartilage against 80AI and 80AII in various lubricants.

Are there any patho-anatomical features that might predispose to primary knee OA? We investigated the 3D geometry of the load bearing zones of both distal femur and proximal tibias, in varus, straight and valgus knees. We then correlated these findings with the location of wear patches measured intra-operatively.

Patients presenting with knee pain were recruited following ethics approval and consent. Hips, knees and ankles were CT-ed. Straight and Rosenburg weight bearing X-Rays were obtained. Excluded were: Ahlbäck grade “>1”, previous fractures, bone surgery, deformities, and any known secondary causes of OA. 72 knees were eligible. 3D models were constructed using Mimics (Materialise Inc, Belgium) and femurs oriented to a standard reference frame. Femoral condyle Extension Facets (EF) were outlined with the aid of gaussian curvature analysis, then best-fit spheres attached to the Extension, as well as Flexion Facets(FF). Resected tibial plateaus from surgery were collected and photographed, and Matlab combined the average tibia plateau wear pattern.

Of the 72 knees (N=72), the mean age was 58, SD=11. 38 were male and 34 female. The average hip-knee-ankle (HKA) angle was 1° varus (SD=4°). Knees were assigned into three groups: valgus, straight or varus based on HKA angle. Root Mean Square (RMS) errors of the medial and lateral extension spheres were 0.4mm and 0.2mm respectively. EF sphere radii measurements were validated with Bland-Altman Plots showing good intra- and interobserver reliability (+/− 1.96 SD). The radii (mm) of the extension spheres were standardised to the medial FF sphere. Radii for the standardised medial EF sphere were as follows; Valgus (M=44.74mm, SD=7.89, n=11), Straight (M=44.63mm, SD=7.23, n=38), Varus (M=50.46mm, SD=8.14, n=23). Ratios of the Medial: Lateral EF Spheres were calculated for the three groups: Valgus (M=1.35, SD=.25, n=11), Straight (M=1.38, SD=.23, n=38), Varus (M=1.6, SD=.38, n=23). Data was analysed with a MANOVA, ANOVA and Fisher's pairwise LSD in SPSS ver 22, reducing the chance of type 1 error. The varus knees extension facets were significantly flatter with a larger radius than the straight or valgus group (p=0.004 and p=0.033) respectively. In the axial view, the medial extension facet centers appear to overlie the tibial wear patch exactly, commonly in the antero-medial aspect of the medial tibial plateau.

For the first time, we have characterised the extension facets of the femoral condyles reliably. Varus knees have a flatter medial EF even before the onset of bony attrition. A flatter EF might lead to menisci extrusion in full extension, and early menisci failure. In addition, the spherical centre of the EF exactly overlies the wear patch on the antero-medial portion of the tibia plateau, suggesting that a flatter medial extension facet may be causally related to the generation of early primary OA in varus knees.

In shoulder arthroplasty, humeral resurfacing or short stem devices rely on the proximal humeral bone for fixation and load transfer. For resurfacing designs, the fixation takes place above the anatomical neck, whilst for short stem designs the resection is made at the anatomical neck and fixation is achieved in the bone distal to that resection. The aim of the study is to investigate the bone density in these proximal areas to provide information for implant design and guidance on appropriate positions to place implant fixation entities.

CT scans of healthy humeri were used to map bone density distribution in the humeral head. CT scans were manually segmented and a solid model of the proximal humerus was discretised into 1mm tetrahedral elements. Each element centroid was then assigned an apparent bone density based on CT scan Grey values. Matlab was used to sort data in spatial groups according to element centroid position to map bone density distribution. The humeral head was divided into twenty 2mm thick slices parallel to the humeral neck starting from the most proximal region of the humeral head to distal regions beneath epiphyseal plate (Fig 1a). Each slice was then radially divided into 30 concentric circles and each circle was angularly divided into 12 regions (Fig 1b). The bone density for each of these regions was calculated by averaging density values of element centroid residing in each region.

Average bone density in each slice indicates that bone density decreases from proximal region to distal regions below the epiphyseal plate and higher bone density was measured proximal to the anatomical neck of the humerus (Fig2). Figure 3 shows that, both above and below the anatomical neck, bone density increases from central to peripheral regions where eventually cortical bone occupies the space. This trend is more pronounced in regions below the anatomical neck and above the epiphyseal plate. In distal slices below the anatomical neck, a higher bone density distribution in inferior (calcar) regions was also observed.

Current generation short stem designs require a resection at the anatomical neck of the humerus and a cruciform keel to fix the implant in the distal bone. In the example in Figure 3, the anatomical neck resection corresponds to the 18 mm slice, with the central cruciform keel engaging between slices 18 mm and 27 mm. The data indicates that this keel should make use of the denser bone by the calcar for fixation, suggesting a crucifix orientation as highlighted in Figure 3. The current generation of proximally fixed humeral components are less invasive than conventional long-stemmed designs, but the disadvantage is that they must achieve fixation over a smaller surface area and with a less advantageous lever arm down the shaft of the humerus. By presenting a spatial density map of the proximal humerus, the current study may help improve fixation of proximally fixed designs, with a simple modification of implant rotational orientation to make use of the denser bone in the calcar region for fixation and load transfer.

Introduction

Survival rates of recent total ankle replacement (TAR) designs are lower than those of other arthroplasty prostheses. Loosening is the primary indication for TAR revisions [NJR, 2014], leading to a complex arthrodesis often involving both the talocrural and subtalar joints. Loosening is often attributed to early implant micromotion, which impedes osseointegration at the bone-implant interface, thereby hampering fixation [Soballe, 1993]. Micromotion of TAR prostheses has been assessed to evaluate the stability of the bone-implant interface by means of biomechanical testing [McInnes et al., 2014]. The aim of this study was to utilise computational modelling to complement the existing data by providing a detailed model of micromotion at the bone-implant interface for a range of popular implant designs, and investigate the effects of implant misalignment during surgery.

Revision knee prostheses are often augmented with intramedullary stems to provide stability following bone loss. However, there are concerns with the use of such stems, including loosening caused by strain-shielding, end-of-stem pain, and removal of healthy bone surrounding the medullary canal. Extracortical fixation plates may present an alternative. The aim of the study was to quantitatively evaluate and compare strain-shielding in the tibia following implantation of a knee replacement component augmented with either a conventional intramedullary stem (design1), or extracortical plates (design2) on the medial and lateral surfaces.

Eight composite synthetic tibiae were implanted with one of the two designs, painted with a speckle pattern, loaded in axial compression (peak 2.5 kN) using a materials test machine, and imaged with a 5-megapixel digital image correlation (DIC) system throughout loading. Bone loss was simulated in all models by removing a volume of metaphyseal bone. For four tibiae, the tibial tray was augmented with a cemented stem (∼150 mm). The others were augmented by extracortical plates (maximum 90 mm long) along the medial and lateral surfaces (Fig. 1). Strains were computed using an ARAMIS 5M software system between loaded and unloaded states in the longitudinal direction, for the medial, posterior and lateral surfaces of the tibiae. Strains were checked locally by use of strain gauge rosettes at three levels on medial, lateral and posterior aspects.

The bone strains measured on the posterior surfaces were reported in three regions; proximal (0–70 mm, where the medial extracortical plate lies), middle (70–130 mm, the stem is present but not the extracortical plates), and distal (130–200 mm, beyond the stem). Mean longitudinal strains for both implant types were comparable in the distal region, and were greater than in the other regions (Fig 2). The mean strains differed considerably in the middle region: 565–715 μstrain with stemmed components 1050–1155 μstrain with plated components. Strains followed a similar pattern in the proximal region, particularly very close (20 mm) to the tibial tray component, where the stemmed component bones (775 ± 160 μstrain) displayed less surface strain than the plated component bones (1210 ± 180 μstrain).

Strain-shielding was observed for both designs. The side plates were shorter than the intramedullary rods, so the region of the bone distal to the plates was not strain-shielded, while the same region was strain-shielded when a stemmed component was implanted. It was also shown that in the region of bone just distal of the tibial tray component, design1 shielded the bone from strain 56% more on average than design2. From these results, it can be speculated that the use of extracortical plate rather than intramedullary stems may lead to improved long-term results of revision TKA, assuming the plates and screws provide adequate stability. The extramedullary fixation system preserves more bone than IM fixation, and has the advantage of allowing use of primary TKA components, cemented over the subframe. Similar components have been developed for the femur.

Background

Total Shoulder Arthroplasty (TSA) has been shown to improve the function and pain of patients with severe degeneration. Recently, TSA has been of interest for younger patients with higher post-operative expectations; however, they are treated using traditional surgical approaches and techniques, which, although amenable to the elderly population, may not achieve acceptable results with this new demographic. Specifically, to achieve sufficient visualization, traditional TSA uses the highly invasive deltopectoral approach that detaches the subscapularis, which can significantly limit post-operative healing and function. To address these concerns, we have developed a novel surgical approach, and guidance and instrumentation system (for short-stemmed/stemless TSA) that minimize muscle disruption and aim to optimize implantation accuracy.

Most glenoid implants rely on centrally located large fixation features to avoid perforation of the glenoid vault in its peripheral regions [1]. Upon revision of such components there may not be enough bone left for the reinsertion of an anatomical prosthesis, resulting in a large cavity that resembles a sink hole. Multiple press-fit small pegs would allow for less bone resection and strong anchoring in the stiffer and denser peripheral subchondral bone [2], whilst producing a more uniform stress distribution and increased shear resistance per unit volume [3] and avoiding the complications from the use of bone cement. This study assessed the best combination of anchoring strength, assessed as the ratio between push in and pull out forces (Pin/Pout), and spring-back, measured as the elastic displacement immediately after insertion, for five different small press-fitted peg configurations (Figure 1, left) manufactured out of UHMWPE cylinders (5 mm diameter and length).

16 specimens for each configuration were tested in two types of Sawbones solid bone substitute: hard (40 PCF, 0.64 g/cm3, worst-case scenario of Pin) and soft (15 PCF, 0.24 g/cm3, worst-case scenario of spring-back and Pout). Two different interference-fits, Ø, were studied by drilling holes with 4.7 mm and 4.5 mm diameter (Ø 0.3 and Ø 0.5, respectively). A maximum Pin per peg of 50 N was defined, in order to avoid fracture of the glenoid bone during insertion of multiple pegs. The peg specimens were mounted into the single-axis screw-driven Instron through a threaded fixture. A schematic of the experimental set up is made available (Figure 1, centre). The peg was pushed in vertically for a maximum of 5 mm at a 1 mm/s rate, under displacement control, recording Pin. The spring-back effect was assessed by switching to load control and reducing the load to zero. The peg was then pulled out at a rate of 1 mm/s, recording Pout. The test profile is depicted in Figure 1 (right).

Average Pout/Pin, spring back (in mm) and force-displacement curves for all 80 specimens tested are shown in Figure 2. These were split into groups according to the type of bone substitute and interference-fit, with the right column showing the average values for the Pin. High repeatability among samples of the same configuration tested is noted. Configurations #1, #3 and #5 all exceed the maximum Pin per peg for at least one type of bone. Configuration #2 has the lowest Pin of all (best thread aspect ratio), followed by configuration #4 (thinner threads). The peg configurations #4 and #2 had the highest Pin/Pout. The peg configurations with lowest spring-back after insertion were configuration #2 and #4. Interference fit of Ø 0.3 mm was shown to reduce Pin below maximum limit of 50 N without great influence in spring-back.

This in-vitro study finds which hip joint soft tissues act as primary and secondary passive internal and external rotation restraints so that informed decisions can be made about which soft tissues should be preserved or repaired during hip surgery. The capsular ligaments provide primary hip rotation restraint through a complete hip range of motion protecting the labrum from impingement. The labrum and ligamentum teres only provided secondary stability in a limited number of positions. Within the capsule, the iliofemoral lateral arm and ischiofemoral ligaments were primary restraints in two-thirds of the positions tested and so preservation/repair of these tissues should be a priority to prevent excessive hip rotation and subsequent impingement/instability for both the native hip and after hip arthroplasty.

Common post-operative problems in shoulder arthroplasty such as glenoid loosening and joint instability can be reduced by improvements in glenoid design shape, material choice and fixation method [1]. Innovation in shoulder replacement is usually carried out by introducing incremental changes to functioning implants [2], possibly overlooking other successful design combinations.

We propose an automated framework for parametric analysis of implant design in order to efficiently assess different possible glenoid configurations. Parametric variations of reference geometries of a glenoid implant were automatically generated in SolidWorks. The different implants were aligned and implanted with repeatability using Rhino. The glenoid-bone models were meshed in Abaqus, and boundary conditions and loading applied via a custom-made Python script. Finally, another MATLAB script integrated and automated the different steps, extracted and analysed the results.

This study compared the influence of reference shape (keel vs. 2-pegged) and material on the von Mises stresses and tensile and compressive strains of glenoid components with bearing surface thickness and fixation feature width of 3, 4, 5 or 6 mm. A total of 96 different glenoid geometries were implanted into a bone cube (E = 300 MPa, ν = 0.3). Fixed boundary conditions were applied at the distal surface of the cube and a contact force of 1000 N was distributed between the central nodes on the bearing surface. The implants were assigned UHMWPE (E = 1 GPa, ν = 0.46), Vitamin E PE (E = 800 MPa, ν = 0.46), CFR-PEEK (E = 18 GPa, ν = 0.41) or PCU (E = 2 GPa, ν = 0.38) material properties and the bone-implant surface was tied (Figure 1). The von Mises stresses, compressive and tensile strains for the different models were extracted. The influence of design parameters in the mechanical environment of the implant could be assessed. In this particular example, the 95^th percentile values of the tensile and compressive strains induced by modifications in reference shape could be evaluated for all the different geometries simultaneously in form of radar plots. 2-pegged geometries (green) consistently produced lower tensile and compressive strains than the keeled (blue) configurations (Figure 2). Vitamin E PE and PCU glenoids also produced lower maximum von Mises stresses values than CFR-PEEK and UHMWPE designs (Figure 3).

The developed method allows for simple, direct, rapid and repeatable comparison of different design features, material choices or fixation methods by analysing how they influence the mechanical environment of the bone surrounding the implant. Such tool can provide invaluable insight in implant design optimisation by screening through multiple potential design modifications at an early design evaluation stage and highlighting the best performing combinations. Future work will introduce physiological bone geometries and loading, a wider variety of reference geometries and fixation features, and look at bone/interface strength and osteointegration predictions.

Hip impingement causes clinical problems for both the native hip, where labral or chondral damage can cause severe pain, and in the replaced hip, where subluxation can cause squeaking/metallosis through edge loading, or can cause dislocation. There is much research into bony/prosthetic hard impingements showing that anatomical variation/component mal-positioning can increase the risk of impingement. However, there is a lack of basic science describing the role of the hip capsule and its intertwined ligaments in restraining range of motion, ROM, and so it is unclear if careful preservation/repair of the capsular ligaments would offer clinical benefits to young adults, or could also help prevent edge loading in addition to reducing the postoperative dislocation rate in older adults.

This in-vitro study quantifies the ROM where the capsule passively stabilises the hip and compares this to hip kinematics during daily activities at risk for hip subluxation.

Ten cadaveric left hips were skeletonised preserving the joint capsule and mounted in a testing rig that allowed application of loads, torques and rotations in all six-degrees of freedom (Figure 1). At 27 positions encompassing a complete hip ROM, the passive rotation resistance of each hip was recorded. The gradient of the torque-rotation profiles was used to quantify where the capsule is taut/slack and after resecting the capsule, where labral impingement occur. The ROM measurements were compared against hip kinematics from daily activities.

The capsule tightly restrains the hip in full flexion/extension with large slack regions in mid-flexion. Whilst ligament recruitment varies throughout hip ROM, the magnitude of restraint provided is constant (0.82 ± 0.31 Nm/degree). This restraint acts to prevent or reduce loading of the labrum in the native hip (Figure 2). The measured passive rotational stability envelope is less than clinical ROM measurements indicating the capsule does provide restraint to the joint within a relevant ROM. Activities such as pivoting, stooping, shoe tying and rolling over in bed all would recruit the capsular ligaments in a stabilising role.

The fine-tuned anatomy of the hip capsule provides a consistent contribution to hip rotational restraint within a functionally relevant ROM for normal activities protecting the hip against impingement. Capsulotomy should be kept to a minimum and routinely repaired in the native hip to maintain natural hip mechanics. Restoring its native function following hip replacement surgery may provide a method to prevent subluxation and edge loading in the replaced hip.

Instability is reported to account for around 20% of early TKR revisions. The concept of restoring the “Envelope of Laxity” (EoL) mandates a balanced knee through a continuous arc of functional movement. We therefore hypothesised that a single radius (SR) design should confer this stability since it has been proposed that the SR promotes normal medial collateral ligament (MCL) function with isometric stability throughout the full arc of motion.

Our aim was to characterise the EoL and stability offered by a SR cruciate retaining (CR)-TKR, which maintains a SR from 10–110° flexion. This was compared with that of the native knee throughout the arc of flexion in terms of anterior, varus/valgus and internal/ external laxity to assess whether a SR CR-TKR design can mimic normal knee joint kinematics and stability.

Eight fresh frozen cadaveric lower limbs were physiologically loaded on a custom jig. The operating surgeon performed anterior drawer, varus/ valgus and internal/external rotation tests to determine ‘maximum’ displacements in 1) native knee and 2) single radius CR-TKR (Stryker Triathlon) at 0°, 30°, 60°, 90° and 110° flexion. Displacements were recorded using computer navigation. Significance was determined by linear modelling (p≤0.05).

The key finding of this work was that the EoL offered by the SR CR-TKR was largely equivalent to that of the native knee from 0–110°. The EoL increased significantly with flexion angle for both native and replaced knees. Overall, after TKR anterior laxity was comparable with the native knee, whilst total varus-valgus and internal-external rotational laxities reduced by only 1°. However, separated varus and valgus laxities at 110° significantly increased after TKR as did anterior laxity at 30° flexion.

In conclusion, the overall EoL offered by the SR CR-TKR is comparable to that of the native knee. In the absence of soft tissue deficiency, the implant appears to offer reliable and reproducible stability throughout the functional range of movement, with exception of anterior laxity at 30° and varus and valgus laxity when the knee approaches high flexion. These shortcomings should offer scope for future work.

Introduction:

Varus alignment of the knee is common in patients undergoing unicondylar knee replacement. To measure the geometry and morphology of these knees is to know whether a single unicondylar knee implant design is suitable for all patients, i.e. for patients with varus deformity and those without. The aim of this study was to identify any significant differences between normal and varus knees that may influence unicondylar implant design for the latter group.

Background

High functional aspirations and an active ageing population equate to a growing number of patients awaiting hip arthroplasty demanding superior biomechanical function. The purpose of this study was to compare the biomechanics of top walking speed between two commonly used hip arthroplasty procedures to determine if a performance advantage existed.