The main purpose of the present study was to determine long-term implant fixation of 15 unicompartmental knee arthroplasty (UKAs) with an all-poly tibial component using Roentgen stereophotogrammetric analysis (RSA) at a mean 10-year follow-up. The secondary purpose was to investigate whether the progressive loss of implant's fixation correlates with a reduction in Knee society score (KSS). Fifteen non-consecutive patients with primary knee osteoarthritis received a UKA with an all-poly tibial component were assessed using KSS scores pre-operatively and post-operatively and RSA on day 2 after surgery, then at 3, 6, and 12 months and yearly thereafter. The mean last follow-up was 10 years. An increase in maximum total point motion (MTPM) values from 6 months to 1 year post-operatively was found respect to post-operative reference. Implants’ displacement values were always 2 mm during the first 6 months, and then, two different trends were noticed in revised and non-revised implants. MTPM increase between 1 and 2 years of follow-up in non-revised UKAs was always 0.2 mm, whereas it was [0.2 mm in revised UKAs. A linear and negative correlation with statistical significance was found between MTPM and both clinical and functional KSS scores (p 0.001). Also in a long-term follow-up evaluation, RSA is an effective tool to predict functional results after an all-poly UKA providing also a relevant predictive value at 1 year follow-up, and this can be very useful for both patients and surgeons.
Roentgen Stereophotogrammetric Analysis (RSA) represents nowadays an excellent solution for high-accuracy fixation measurement of UKAs. To date, to the best of our knowledge, no previous study has investigated long-term micromotions of a UKA using RSA. For this reason, the main purpose of the present study is to determine long-term implant fixation of 15 UKAs with all-poly tibial component using RSA at a mean 10-years follow-up. The secondary purpose was to investigate whether the progressive loss of implant's fixation correlates with a reduction of KSS scores. Fifteen non-consecutive patients with primary knee osteoarthritis received a UKA with an all-poly tibial component between January 1995 and April 2003 in the Authors' institution. Pre and post-operative KSS scores were recorded. RSA evaluation was performed on day 2 after surgery, than at 3, 6, and 12 months and yearly thereafter. The patients were evaluated with a mean follow-up of 10 years.Background
Methods
Over the last years research has focused on attempts to achieve better fixation in knee prosthesis by improving cementing techniques and prosthetic designs; thus prosthetic fixation is critical while loosening at the bone-cement interface is still an important matter for the orthopaedic surgeon. In the present study, we evaluated in vivo displacement of both total and unicompartimental knee prosthesis implanted on 73 patients by collecting data from RSA associated to the conventional X-rays and clinical follow-ups; we were able to reach only 18 patients (2 males and 16 females) for the actual follow up because of death or other diseases occurred on the other patients over the years (mean age at the time of surgery 68, range 27–83).Introdution:
Material and Method:
The Rizzoli Orthopaedic Institute has been involved in RSA since 1998. During last 25 years, several investigations have been carried on to evaluate both implant fixation and poliethylene deformation in unicompartmental and total knee replacements. Nevertheless, RSA has also been used to investigate the relative micromotions and the kinematic modifications in cadaveric models of ankles with ligamentous injuries. RSA evaluation has demonstrated that in a particular TKR implant, with mobile half-bearings, the threshold for loosening was 1.3° for rotation about the longitudinal axis and 0.5 mm for medio-lateral translation. Moreover, RSA has revealed cold flow to be concentrated in the posterior region of the medial half-bearing. This has lead to further improvement in polyethilene and implant design. RSA has also demonstrated that in all-poly tibial UKR, poliwear does not impair tibial component fixation and that deformation of the all-poly tibial component is strictly correlated to implant loosening. Moreover, RSA has been used to investigate stress-inducible displacement of the tibial component in all-poly UKRs. It has been demonstrated that rotations around the transverse axis of the knee joint are the most common form of stress-inducible displacement, while stress-induced translations are negligible. Moreover, stress-inducible translational displacement has reached significantly higher values for those patients with unexplained painful UKR, despite no sign of loosening on conventional radiographic and standard supine RSA evaluations. Further application of RSA has focused on the kinematic evaluation of poliethilene motion pattern modifications throughout a 3 years follow-up period after a mobile-bearing TKR. Patients have been investigated in weight-bearing conditions and results demonstrated that longitudinal rotations and medio-lateral translations tended to increase at last follow-up, while sagittal translations dod not show any significant modification over time. At present, a new device has been installed at the Istituto Ortopedico Rizzoli. It was specifically designed and made for RSA, static and fluoroscopic. This device can work both in mono- and bi-planar configuration as required by the RSA protocols. Moreover it is able to acquire image stacks in order to study the in-vivo and real time kinematics of a joint. he theoretical biomechanical resolution of a static RSA followup tests is 0.2 mm for translation and 0.3° for rotation. In fluoroscopic configuration the theoretical resolution is 1 mm for translation and 1° for rotations, depending on the used frame rate and on the joint movement speed. A kinematic comparison of different prosthetic designs is currently ongoing, to evaluate different motion patterns under dynamic weight-bearing conditions and to compare them with passive kinematics acquired intra-operatively using a navigation system.
