Roentgen stereophotogrammetric analysis (RSA) is the most accurate radiographic technique for the assessment of three-dimensional micromotion in joints. RSA has been used previously to study the kinematics of the anterior cruciate ligament (ACL)-deficient knee and to measure knee laxity after bone-tendon-bone (BTB) reconstructions. There is no published evidence on its use in assessing hamstring grafts in vivo, in comparing hamstring versus BTB reconstruction, or in-depth analyses of graft performance. The aim of this project was to use RSA to measure laxity in both BTB and hamstring reconstructions, and to attempt a detailed analysis of graft behaviour in both reconstructions, with particular attention to graft stretching and slippage of the bony attachments. A prospective study was undertaken on 14 patients who underwent ACL reconstruction. Seven had BTB reconstruction, and seven had four-stranded semitendinosus/gracilis (STG). Tantalum markers were inserted at the time of surgery, into distal femur and proximal tibia, and also directly into the graft itself. Stress radiographs (90N anterior and 90N posterior force) were taken early post-operatively, and then at 6 weeks, and 3, 6 and 12 months. In addition to measuring total anteroposterior knee laxity, a detailed analysis of the graft itself was possible. The BTB grafts had stretched by an average of 1.54%, and the bone plugs had migrated by 0.50 mm at the femoral end and by 0.61mm at the tibial end. The hamstring grafts had stretched on average 3.94%, and the intraosseous ends had migrated by 3.96mm at the femoral end and by 7.10mm at the tibial end. This is believed to be the most detailed application of RSA in analysing the performance of the two commonly used grafts in ACL reconstruction. Details such as graft stretching and fixation slippage have not been available previously; the data obtained in this study may have implications for clinical practice.
Controversy still exists in the literature regarding efficacy and usefulness of CASN in knee arthroplasty. However, obsession with basic alignments and proper correction of mechanical axes fails to recognise the full future potential of CASN which seems to lie in enhanced dynamic assessment. Basic dynamics usually at least includes intraoperative assessment of limb alignments, flexion-extension gap balancing and simple testing through ranges of motion. However our upgraded CASN system (Brainlab) is also capable of enhanced assessment not only including the provision of data on initial to final alignments but also contact point observations. The system can also perform an enhanced ‘Range Of Motion’ (ROM) analysis including observation of epicondylar axis motion, valgus and varus, antero-posterior shifts as well as flexion and extension gaps. Tracking values for both tibiofemoral and patellofemoral motion have also been obtained after performing registration of the prosthetic trochlea. Observations were then made using a set of standardised dynamic tests. Firstly, the lower leg was placed in neutral alignment and the knee put through a flexionextension cycle. Secondly the test was repeated but with the lower leg being placed into varus and internal rotation. The third test was performed with the lower leg in valgus and external rotation. We have been able to carry out these observations in a limited case series of 15 total knee arthroplasties and have found it possible to observe and quantify marked intra-operative variation in the stability characteristics of the implanted joints before corrections have been made and final assessments performed. Indeed contact point observation has found several cases of edge loading before corrections have been made. Also ROM analysis has demonstrated the ability of the system in other cases to observe and then make necessary adjustments of implant positions and ligament balance which alter the amounts of antero-posterior and lateral translations. In this way paradoxical antero-posterior and larger rotational movements have been minimised. Cases where conversion to posterior stabilisation has been necessary have been encountered. Also patellar tracking has been observed during such dynamic tests and appropriate adjustments made to components and soft tissue balancing. Although numbers in this case series are small, it has been possible to begin to observe, classify and quantify patterns of instability intra-operatively using simple stress tests. Such enhanced intra-operative information may in future make it possible to create algorithms for logical adjustments to ligament balance, component sizes, types and positions. In this way CASN becomes a more useful tool.
Our aim was to determine the effects of tibial component malrotation and posterior slope on knee kinematics following Scorpio cruciate retaining total knee replacement in cadaver specimens. The movements of the hip, thigh and lower leg were monitored in 3D using a validated infra-red Computer Navigation System via bone implanted trackers. Ten normal comparable cadaver specimens were mounted in a custom rig allowing 3D assessment of kinematics under various loading conditions. The specimens then underwent Navigated TKR as per normal operating surgical protocols however an augmented tibial component was implanted. This allowed the researchers to precisely modify the rotation of the tibial component around its predetermined central axis, as well as to alter the posterior slope of the component. A pneumatic cylinder was used to provide a simulated quadriceps extension force while the knee was tested with a variety of applied loads including anterior and posterior draw, abduction and adduction, internal and external rotation. TKR kinematics are significantly different from those of the native knee (p<
0.05). Increasing tibial posterior slope resulted in an incremental posterior position of the femur (p<
0.05), deviation of the neutral path of motion (p<
0.05) and alteration of the normal AP envelope of laxity (p<
0.05). Tibial component malrotations over 10 degrees resulted in increasing deviations of the neutral movement path of motion (p<
0.05) without significantly affecting the envelope of laxity. Tibial component malrotations of more than 10 degrees, when combined with a posterior slope of six degrees or more, resulted in prosthetic subluxation under certain loading conditions. This study has demonstrated significant differences in knee kinematics before and after total knee implantation. Increasing values of internal and external rotation, as well as posterior slope of the tibial tray resulted in further deviations of total knee kinematics from normal by altering the neutral path of motion and the soft tissue envelope, with combined misalignments resulting in the greatest deviations from normal with prosthetic subluxation in some cases. Deviations from normal kinematics may result in increased ligament tension and incongruence or dysfunction of the component articulations, with the generation of sheer forces in the gait cycle. These may contribute to premature wear and loosening. Surgeons should be aware of this when considering the addition of posterior slope or assessing tibial component positioning in TKR.
Differing descriptions of patellar motion relative to the femur have resulted from many in-vitro and in-vivo studies. The aim of this study was to examine the tracking behaviour of the patella. We hypothesized that patellar kinematics would correlate to the trochlear geometry and that differing previous descriptions could be reconciled by accounting for differing alignments of measurement axes. Seven normal fresh-frozen knees were CT scanned and their kinematics with quadriceps loading was measured by an optical tracker system and calculated in relation to the previously-established femoral axes. CT scans were used to reliably define frames of reference for the femur, tibia and the patella. A novel trochlear axis was defined, between the centres of best-fit medial and lateral trochlear articular surfaces spheres. The path of the centre of the patella was circular and uniplanar (RMS error 0.3mm) above 16°±3° knee flexion. The distal end of the median ridge of the patella entered the groove at 6° knee flexion, and the midpoint at 22°. This circle was aligned 6.4° ± 1.6° (mean± SD) from the femoral anatomical axis, 91.2°±3.4° from the epicondylar axis, and 88.3°±3° from the trochlear axis, in the coronal plane. In the transverse plane it was 91.2°±3.4° and 88.3°±3° from the epicondylar and trochlear axes. Manipulation of the data to different axis alignments showed that differing previously-published data could be reconciled. When the anatomic axis of the femur was used to align the coordinates, there was an initial medial and then a lateral translation. Comparing this with the uniplanar and circular path of the center of the patella, it shows that the orientation of the femoral coordinate system affects the description of the patellar medial-lateral translation. This study has shown the effect of using different coordinate systems on reporting the patellar translation. Choosing a femoral reference that is more in line with the plane of the circular path of motion and the trochlear groove in the coronal plane diminishes the reported subsequent lateral translation of the patella. Once the frame of reference had been aligned to the trochlear axis, there was minimum medial-lateral translation of the patella.
Kinematics were different after TKR. Increasing posterior slope resulted in increasing posterior position of the femur, particularly at maximum flexion. Posterior slope also resulted in a deviation of the neutral path of motion and alteration of the normal envelope of laxity. Tibial component malrotations over 5 degrees resulted in deviations of the neutral path of motion without affecting the envelope of laxity. Combined malrotations over 10 degrees with posterior slopes over 6 degrees resulting in prosthetic subluxation under certain loading conditions.
In the sagital view, the centre of the circle is offset by 21mm (S.D.3mm) at an angle of 67° (S.D. 7°) from a line connecting the midpoint between the centres of the femoral condyles and the femoral head centre. On either end of this line, the articular surface of the trochlea can be fitted to spheres of radius 30mm (S.D. 6mm) laterally and 27mm (S.D. 5mm) medially, with an rms of 0.4mm.
This can be of use in planning and performing joint reconstruction and have implications for the design of patello-femoral replacements and the rules governing their position.
Acetabular centre positioning in the pelvis has a profound effect on hip joint function. The force–and moment-generating capacities of the hip muscles are highly sensitive to the location of the hip centre. We describe a novel 3D CT-based system that provides a scaled frame of reference (FOR) defining the hip centre coordinates in relation to easily identifiable pelvic anatomic landmarks. This FOR is more specific than the anterior pelvic plane (APP) alone, giving depth, height and width to the pelvis for both men and women under-going hip surgery. CT scans of 22 normal hips were analysed. There were 14 female and 8 male hips. The APP was used as the basis of the coordinate system with the origin set at the right anterior superior iliac spine. After aligning the pelvis with the APP, the pelvic horizontal dimension (Dx) was defined as the distance between the most lateral points on the iliac crests, and its vertical dimension (Dy) was the distance between the highest point on the iliac wing and the lowest point on ischial tuberosity. The pelvic depth (Dz) was defined as the horizontal distance between the posterior superior iliac spine and the ipsilateral ASIS. The ratios of the hip centre’s x, y, and z coordinates to their corresponding pelvic dimensions (Cx/Dx, Cy/Dy, Cz,Dz) were calculated. The results were analysed for men and women. For a given individual the hip centre coordinates can be derived from pelvic landmarks. We have found that the mean Cx/Dx measured 0.09 ± 0.02 (0.10 for males, 0.08 for females), Cy/Dy was 0.33 ± 0.02 (0.30 for males, 0.35 for females), and Cz/Dz was 0.37 ± 0.02 (0.39 for males and 0.36 for females). There was a statistically significant gender difference in Cy/Dy (p=0.0001) and Cz/Dz (p=0.03), but not in Cx/Dx (p=0.17). Anteversion for the male hips averaged 19° ± 3°, and for the female hips it was 26° ± 5°. Inclination measured 56° ± 1° for the males and 55° ± 4° for the females. Reliability testing showed a mean intra-class correlation coefficient of 0.95. Bland-Altman plots showed a good inter-observer agreement. This method relies on a small number of anatomical points that are easily identifiable. The fairly constant relationship between the centre coordinates and pelvic dimensions allows derivation of the hip centre position from those dimensions. Even in this small group, it is apparent that there is a difference between the sexes in all three dimensions. Without the need for detailed imaging, the pelvic points allow the surgeon to scale the patient’s pelvis and thereby know within a few millimetres the ‘normal’ position of the acetabulum for both men and women. This knowledge may be of benefit when planning or undertaking reconstructive hip surgery especially in patients with hip dysplasia or bilateral hip disease where there is no reference available for planning the surgery.
A recurrence of objectively measured knee laxity after anterior cruciate ligament (ACL) reconstruction has previously been reported in various papers; the purpose of this study was to accurately measure in vivo knee laxity after both bone-tendon-bone (BTB) and hamstring reconstruction using radiostereometric analysis (RSA), and to differentiate between graft fixation slippage and graft stretching and their relative contributions to any increase in laxity. Twenty patients were studied prospectively after ACL reconstruction. Ten had been operated on using BTB grafts, and ten using hamstring (four-stranded semitendinosus/gracilis) grafts. Tantalum markers were inserted in the distal femur, proximal tibia and into the graft itself. (RSA) was used to measure sagittal laxity, graft stretching and fixation slippage early post-operatively, and then at intervals up to 1 year. A steady increase in total anteroposterior laxity was found in both groups over the year. For the BTB group, total mean slippage of the bone plugs increased to 1.28 mm at 1 year. For the hamstring group, the tunnel attachments had slipped by a total of 6.82 mm. More stretching was found for the hamstrings grafts than for the BTB grafts and the amount of stretching increased significantly with time post-surgery. The hamstring grafts stretched by a mean of 4.18%, the BTB grafts by 1.18%. This is believed to be the most detailed application of RSA in analysing the performance of the two commonly used grafts in ACL reconstruction. Details such as graft stretching and fixation slippage have not been available previously; the data obtained in this study may have implications for clinical practice.
On either end of this line, the articular surface of the trochlea can be fitted to spheres of radius 30mm (stdev 6mm) laterally and 27mm (stdev 5mm) and an rms of 0.4mm medially. The centres of the circle and the two spheres fall on a line with an rms of 1.1mm. The anterior and proximal patellar planes could be described as flat surfaces (rms of 0.4 and 0.3mm). The median ridge could be described as a straight line (rms of 0.2mm). The angle between planes was 112° (stdev 5°); the average angle between the proximal plane and the line on the medial ridge was 62° (stdev4°). The functional centre of the patella was defined as a point in the centre of 2 planes orthogonal to the sagital plane from the midpoint between the most proximal and most distal points on the median ridge. The length, width and thickness of the patellae were measured at 22mm +/−4mm, 47mm +/− 3mm and 24 mm+/− 2 mm.
Patellofemoral complications in total knee arthroplasty (TKA) are common. Patellar tracking can be adversely affected by component positioning, soft tissue imbalance and bony deformity. Lateral patellar release rates reported in the literature vary from 6– 40%. Computer assisted surgery has largely been confined to the tibio-femoral component of total knee replacement. However, with recently developed software, it can be used to visualise and quantify patellar tracking and thus guide the precise extent and site of lateral patellar release. The aim of this early study was to define the diagnostic envelope for identification and quantisation of patella maltracking using a current generation patella navigation system. Our previous prospective analysis of 100 patients undergoing primary TKA identified pre-operative radiographic indices that correlate with maltracking of the patellofemoral joint. 20 cases were subsequently selected for computer assisted total knee replacement surgery. The navigation system (Vector Vision (BrainLab) version 1.6) was used to achieve accurate alignment and position of the femoral and tibial components. All knee replacements were performed using a posterior cruciate-retaining prosthesis. The femoral component was of a ‘patella-friendly’ design with inbuilt 3 degrees external rotation, and the patella was resurfaced in all cases with a biconvex inlay patellar prosthesis. Patellar tracking was assessed intra-operatively using an additional patellar array and patella tracking-specific software. Real-time displays of patella shift, tilt, rotation and circle radii during multiple flexion-extension cycles were obtained. Where necessary, an ‘outside-to-in’ release of the lateral retinacular complex was performed. The navigation system was used to provide contemporaneous feedback on the effect of the soft tissue releases on the tracking characteristics of the patella component on the prosthetic trochlea. Primary outcomes included the sensitivity and specificity of the system for peri-operative patella maltracking; secondary outcomes included the definition of interventional endpoints and correlation of intra-operative tracking data with post-operative x-rays. The demographic data for the 20 patients enrolled in this study was essentially unremarkable. As compared to standard intra-operative clinical evaluation of patella tracking, the computer navigation system is equally sensitive and specific, and it can potentially detect more subtle instances of maltracking that may elude conventional clinical evaluation. We present patterns of patellar tracking during the surgery for patient with and without pre-operative patellar maltracking. However, the significance of this is unknown without longer-term outcome data. Patella shift abnormalities that were detected by the system, but not tilt, correlated with clinical judgement of patella maltracking (p<
0.05). Soft tissue balancing of the patella can now be performed by observing precise changes in shift and tilt. This can be as important as component alignment for optimising patellar tracking and minimising patellofemoral complications.
Patellofemoral symptoms are a prominent cause of dissatisfaction following knee arthroplasty. This may relate to difficulty in knowing where to resect the bone and in placing prosthetic components to reproduce the anatomy accurately. This study developed geometrical data to facilitate these procedures during TKR. Thirty CT scans of patients above the age of 55 without patellofemoral disease were performed. Three dimensional images were reconstructed using computer software that enabled manipulation of these images and measurements to be taken. These models allowed the shape of the patella to be modelled, its size and the track it takes in the normal trochlea. The anterior and proximal patellar planes could be described as flat surfaces with an rms of 0.4 and 0.3mm. The angle between these planes was 112° (stdev 5°). The median ridge of the articular surface was a straight line with an rms of 0.2mm and the average angle between the anterior plane and this line was 12° (stdev4°). The angle between the anterior plane and a line fitted to the posterior aspect of the apex of the patella was 56° (stdev 2°). Having oriented the patella with the proximal plane vertical, the distal pole of the patella was within 2mm of the same sagittal plane as the median ridge of the articular surface in all cases. The functional centre of the patella was defined as a point in the centre of 2 planes orthogonal to the sagittal plane at the midpoint between the most proximal and most distal points on the median ridge. In the transverse section this centre was always on the line separating the superficial and deep surfaces of the patella. Also the length, width and thickness of the patellae were measured at 22mm +/−4mm, 47mm +/− 3mm and 24 mm+/− 2 mm. The average ratio of the lateral facet to medial facet width was 1.3 (range 0.8–1.6). The average ratio of the patellar width to thickness was 2.0 (S.D. 0.106, 95%CI 1.96 to 2.03) with a strong correlation(r= 0.89). From this work we have concluded that the anterior and proximal planes of the patella, which will not be affected by the disease, can be defined and used as a frame of reference for the patella, which will be helpful for navigating the patella and restoring its anatomical form in the presence of erosive changes. The patella has a constant shape, so that its articular surface can be defined in relatively simple terms, and can be referenced off its non articular surface.