Summary Statement

Wear of total knee replacement (TKR) is a clinical concern. This study demonstrated low-conformity moderately cross-linked-polyethylene fixed bearing TKRs showed lower volumetric wear than conventional-polyethylene curved fixed bearing TKRs highlighting potential improvement in TKR performance through design and material selection.

Total knee replacements (TKR) have been the main choice of treatment for alleviating pain and restoring physical function in advanced degenerative osteoarthritis of the knee. Recently, there has been a rising interest in minimally invasive surgery TKR (MIS-TKR). However, accurate restoration of the knee axis presents a great challenge. Patient-specific-instrumented TKR (PSI-TKR) was thus developed to address the issue. However, the efficacy of this new approach has yet to be determined. The purpose of the current study was thus to measure and compare the 3D kinematics of the MIS-TKR and PSI-TKR in vivo during sit-to-stand using a 3D fluoroscopy technology.

Five patients each with MIS-TKR and PSI-TKR participated in the current study with informed written consent. Each subject performed quiet standing to define their own neutral positions and then sit-to-stand while under the surveillance of a bi-planar fluoroscopy system (ALLURA XPER FD, Philips). For the determination of the 3D TKR kinematics, the computer-aided design (CAD) model of the TKR for each subject was obtained from the manufacturer including femoral and tibial components and the plastic insert. At each image frame, the CAD model was registered to the fluoroscopy image via a validated 2D-to-3D registration method. The CAD model of each prosthesis component was embedded with a coordinate system with the origin at the mid-point of the femoral epicondyles, the z-axis directed to the right, the y-axis directed superiorly, and the x-axis directed anteriorly. From the accurately registered poses of the femoral and tibial components, the angles of the TKR were obtained following a z-x-y cardanic rotation sequence, corresponding to flexion/extension, adduction/abduction and internal/external rotation.

During sit-to-stand the patterns and magnitudes of the translations were similar between the MIS-TKR and PSI-TKR groups, with posterior translations ranging from 10–20 mm and proximal translations from 29–31mm. Differences in mediolateral translations existed between the groups but the magnitudes were too small to be clinically significant. For angular kinematics, both groups showed close-to-zero abduction/adduction, but the PSI-TKR group rotated externally from an internally rotated position (10° of internal rotation) to the neutral position, while the MIS-TKR group maintained at an externally rotated position of less than 5° during the movement.

During sit-to-stand both groups showed similar patterns and magnitudes in the translations but significant differences in the angular kinematics existed between the groups. While the MIS-TKR group maintained at an externally rotated position during the movement, the PSI-TKR group showed external rotations during knee extension, a pattern similar to the screw home mechanism in a normal knee, which may be related to more accurate restoration of the knee axis in the PSI-TKR group. A close-to-normal angular motion may be beneficial for maintaining a normal articular contact pattern, which is helpful for the endurance of the TKR. The current study was the first attempt to quantify the kinematic differences between PSI and non-PSI MIS. Further studies to include more subjects will be needed to confirm the current findings. More detailed analysis of the contact patterns is also needed.

Summary Statement

Using current analysis/methodology, new implant technology is unlikely to demonstrate a large enough change in patient function to impact on the cost-effectiveness of the procedure.

The price per total knee replacement (TKR) performed is fixed but the subsequent length of hospital stay (LOS) is variable. The current national average for LOS following TKR is six days. LOS is an important marker of resource consumption, has implications in patient satisfaction, and is used as a marker of hospital quality. The aim of this study was to describe the temporal change in demographics between 2004 and 2009, and to identify intra-operative factors and patient characteristics associated with a prolonged LOS that could be addressed to improve clinical practice. We performed a retrospective cohort review of 184 patients (2004 n=88, 2009 n=96) who underwent primary TKRs at Chorley District General Hospital. The median LOS in 2009 was eight days compared to ten days in 2004, an average of 3.5 days less (p < 0.001). Patients were significantly younger (p < 0.001) in 2009 (median 66 years) compared to 2004 (median 74 years), with both years having a similar female predominance. There was no significant change in the BMI or American Society of Anesthesiologists score between 2004 and 2009. This data suggests that block contracts with the private sector has not influenced the demographics of patients being treated in the NHS. Intra-operative factors including the use of a peripheral nerve block, the surgeon grade, the day of the week the operation was performed, the operation length, and the change in pre- to post-operative haemoglobin were not found to significantly increase the LOS (p = 0.058, p = 0.40, p = 0.092, p = 0.50, p = 0.43 respectively). Cemented TKRs had a median LOS of nine days compared to eight for uncemented implants (p = 0.015). However, patients with a cemented implant were on average 6.2 years older than patients with an uncemented implant (p < 0.001). Using Cox proportional hazard regression modelling, the occurrence of a post-operative complication (p < 0.001), female sex (p = 0.024), advancing age (p = 0.036), and the need for a blood transfusion (p = 0.0056) were the most significant factors for prolonging the LOS. Patients who were given a transfusion stayed a median of 13 days compared to nine for those who did not (p < 0.001). The median pre-operative haemoglobin for those who required a transfusion was 11.85g/dl compared to 13.6g/dl for those who did not (p < 0.001). Being obese or morbidly obese did not significantly prolong the LOS (p = 0.95).

In conclusion, this study highlights significant patient characteristics which are associated with a prolonged LOS following TKR. The relatively low pre-operative haemoglobin in patients requiring a blood transfusion is a potential target for reducing the LOS.

Allogeneic blood transfusion is associated with many complications and significant cost. The RD&E has looked at the use of autologous drains after our study of 100 cases showed an improved post-operative haemoglobin and reduced length of stay. There is a need to identify those patients of increased need for an autologous drain, in order to decrease the frequency of allogeneic transfusion. In 2007 a protocol was drawn up using information from our study of 191 cases which showed an average haemoglobin drop post-operatively of 3.05g/dl and average intra-operative blood loss of 285 ml. This protocol gives the surgeon triggers for autologous drain use; preoperative haemoglobin of <13g/dl, intra-operative blood loss of >400ml, tourniquet use, patient weight <50kg and patients refusing donated blood.

In 2007-08, 65% of a further 275 cases analysed met the triggers for use of an autologous system. The remaining patients received low vacuum drains. Of the 275 patients, only 2 (<1%) of those who did not fulfil the criteria for an autologous drain required allogeneic blood, compared with 43 patients (24%) of those deemed high risk of transfusion, and assigned autologous drains. The protocol was therefore deemed to be successful in identifying those patients who required additional support and expenditure to minimise allogeneic blood transfusion.

Analysis of this data led to recommended changes to the protocol in order to maximise the efficiency of the autologous drain use. In 2010 a further patient cohort studied showed a reduction in allogeneic blood transfusion to <10% of those receiving autologous drains, and an increase to 5% of those with low vacuum drains.

Due to the increased cost of autologous drains (£68) compared with the low vacuum systems (£32), and the cost of allogeneic units at £141, the expenditure per patient was calculated and shown to fall from £92 in 2007 to £78 in the 2010.

In conclusion, this protocol allows the clinician to appropriately target the use of the more expensive autologous drains to those of increased risk of transfusion. This protocol helps to minimise unnecessary allogeneic blood transfusion risks, and this has been shown to be more cost effective.

INTRODUCTION

Useful feedback from a Total Knee Replacement (TKR) can be obtained from post-surgery in-vivo assessments. Dynamic Fluoroscopy and 3D model registration using the method of Banks and Hodge (1996) [1] can be used to measure TKR kinematics to within 1° of rotation and 0.5mm of translation, determine tibio-femoral contact locations and centre of rotation. This procedure also provides an accurate way of quantifying natural knee kinematics and involves registering 3D implant or bone models to a series of 2D fluoroscopic images of a dynamic movement.