The aim of this single blinded randomised control trial study was to assess the accuracy of implantation of components and the clinical outcome and complications with the iNAV electromagnetic navigation system compared with conventional techniques. Secondary outcome measures include Patient reported outcome measures (PROMS) and rates of revision surgery up to 10 years post operatively.

Participants Patients were identified by members of the research team from TKA surgical waiting lists. Patients were invited to participate if they had osteoarthritis of the knee suitable for TKA, were able to provide informed consent, and were aged 18 or over. There were no specific limits imposed on the degree of preoperative coronal or sagittal deformity.

Randomization Overall, 272 patients were screened between July 2007 and August 2010. Of the 272 screened patients, 14 were excluded for other medical reasons, whereas 58 participants decided that they did not want to participate in a research study. The remaining 200 patients were recruited and consented to the study giving a recruitment rate of 74%. Patients were randomized in a 1:1 ratio to either conventional TKA or navigated TKA using a web based computer-generated random number table. Randomization was based on the order of their recruitment and stratified by surgeons to prevent surgeon bias and ensure that similar numbers of patients in each groups were allocated to each surgeon. Randomization was successful in assigning equal preoperative patient demographics between the groups.

Ethical Approval The study was approved by the Glasgow Royal Infirmary Local Ethics Committee and the University of Strathclyde Ethics Committee (07/S0704/6) and approved by the NHS GGC R&D department before commencement of the study.

Surgical Procedure All patients received a cemented posterior stabilized NexGen LPS Flex (Zimmer, Warsaw, Indiana, USA). Participants randomized to the conventional group received a TKA implanted using standard instrumentation, whereas those randomized to the navigated group had surgery using the iNav portable EM navigation system (Zimmer GmbH, Winterthur, Switzerland and Medtronic, Minneapolis, MN, USA). The iNav EM system is imageless and uses small reference frames attached to the femur and tibia which are readily incorporated in the primary incision. There is then a process of joint registration which maps the surface anatomy of the joint. All surgeries including joint surface registration were carried out by, or under the direct supervision of, one of two knee arthroplasty surgeons. Alignment targets were similar in both groups with a neutral HKAA and the aim to implant both femur and tibial components perpendicular to this in the coronal plane.

Ligament balancing was carried out using clinical assessment during the surgical procedure in both groups. In the navigated group, additional information was provided by the system with real-time feedback of the gap in mm between the femoral and tibial component and of the overall HKAA during varus and valgus stress.

Primary outcome measurement Post-operative CT scans were used to determine the accuracy of implantation. CT scan analysis was conducted using Mimics 12.0 software (Materialise, Leuven, Belgium). Measurements of the femoral and tibial component position in the coronal (varus/valgus), sagittal (flexion/extension) and axial (rotational) planes were made. The overall mechanical alignment was also calculated from the addition of the femoral and tibial coronal angles. The combined component rotation was calculated from the addition of the femoral and tibial rotation angles. The rotations were measured using the methods detailed in Berger et al (1998). In the coronal plane the investigators aimed to position both femoral and tibial implants at 90° to the mechanical axis. In the sagittal plane the investigators aimed to position the femoral component with a 5° slope relative to the mechanical axis, in line with the anterior cortex of the distal femur. The tibial component was aimed to be positioned at a 7° slope, as per the manufacturer's guidelines. For femoral rotation the investigators aimed to implant the femoral component in line with the surgical trans-epicondylar axis of the femur. The reference for tibial rotation was a line from the geometric center of the tibia to the center of the tuberosity. Rotational measurements were calculated from a perpendicular line drawn from the posterior surface of the implant. As the tuberosity is 18° externally rotated, the investigators considered an 18° internal rotation of the implant to be a neutral position. (No obvious deformities of the tibia or previous fractures were noted in the study cohort that could have influenced this value.) The investigators considered the desired mechanical axis alignment to be 0° with a range of ± 3°.

Patient-Reported Outcome Measures and Revisions Participants were followed up for 10 years after surgery, with clinical assessments by a blinded independent assessor; range of motion (ROM) was determined using a hand-held goniometer, and knee-specific outcome measures included the American Knee Society Score (AKSS) and Oxford Knee Scores (OKS) and the SF 36 score used as a general health measure (both physical and mental). Revision surgery was also assessed by analyzing the Scottish National Picture Archiving and Communication System. This image archiving system stores all imaging for patients undertaken in the National Health Service (NHS) in Scotland since 2008. It acts as a valuable resource for identifying patients lost to follow-up who have undergone revision surgery in other NHS hospitals in Scotland which may have not been identified.

Statistics A power calculation was performed based in data provided by a randomized controlled trial using infra red optical tracking systems. Bathis et al reported 96% of patients with mechanical leg alignment within 3° of neutral using navigation compared to just 78% with conventional instrumentation . In order to detect a difference of this magnitude with a power of 90% at alpha = 0.05, the investigators would require 82 patients per group, 164 in total. As the primary outcome measure was based on post-operative CT scan the investigators anticipated a higher than average loss to follow-up for the primary outcome measure. The investigators therefore allowed an additional 25% for loss to follow-up, giving a total of 103 patients in each group.

Statistical analysis was performed using SigmaPlot 11.0 (Systat Software Inc). To evaluate differences between the surgical groups either a two sample t test (normally distributed data) or a Mann Whitney test (non parametric data) was performed. A Chi Squared test was used to analysis the male: female ratio. A P value of less that 0.05 was considered significant.

Inclusion Criteria:

• Male or female subjects
• Subjects must be at least 18 years of age and there will be no maximum age limit. The subjects age must be considered suitable by the Investigator for a knee arthroplasty using either of the two systems available in the evaluation.
• Subjects, who are able to give voluntary, written informed consent to participate in this investigation and from whom consent has been obtained.
• Subjects, who, in the opinion of the investigator, are able to understand this investigation, co-operate with the investigation procedures and are willing to return to the hopsital for all the required post-operative follow-ups.
• Subjects who require a knee arthroplasty for primary surgical management of the idiopathic osteoarthritis.
• Subjects, who in the opinion of the Investigator, are considered to be suitable for treatment with a NexGen LPS Flex Total Knee Replacement.

Exclusion Criteria:

• Subjects who, in the opinion of the investigator, have an existing condition that would compromise their participation and follow-up in the study
• Subjects who require revision total knee arthroplasty surgery
• Subjects with any tibial deformity requiring tibial component augmentation.
• Subjects whom, in the opinion of the Investigator, require a constratined prosthesis.
• Subjects with inflammatory polyarthritis.
• Subjects with disorders of the feet, ankles, hips, or spine caising significant abnormal gait or significant pain.
• Subjects with osteoarthritis of the contralateral knee causing significant abnormal gait or significant pain.
• Subjects with poorly functioning contralateral total knee replacement causing significant abnormal gait or significant pain. Subjects with a well functioning contralateral total knee replacement will not be excluded.
• Subjects with neurological conditions affecting movement.
• Subjects with a pathology which, in the opinion of the Investigator, will adversely affect healing.
• Subjects with other disorders which, in the opinion of the Investigator, will/could impair rehabilitation.
• Contra-indications for the use of the device, as detailed in the package insert.
• Women who are pregnant
• Subjects who are known drug or alcohol abusers or with psychological disorders that could effect follow-up care or treatment outcomes.
• Subjects who are currently invovled in another clinical study with an investigational product.
• Subjects who are currently invovled in any injury litigation claims.