Computer navigation systems enable precise measurement and intra- operative knee range of movement analysis. We present a series of five knees that demonstrated unusual kinematics.

Five of 80 computer navigated knee replacements that were part of a prospective randomised trial were found to have unusual joint lines. Range of motion assessment was performed with computer assisted navigation after exposure and registration of bony landmarks and before bony resection was commenced. This revealed valgus alignment in extension that drifted into varus with knee flexion. We referred to these unusual patterns as ‘oblique joint lines’.

The data from the navigation log files of these five knees was analysed in detail. Average age of patients in this series was 68years and all were female. The average pre- operative angle between femoral axis and distal femoral articular surface was 101 degrees. All five knees had a tibial varus with average angle between the tibial axis and articular surface being 85 degrees. In two knees, more bone was resected from the medial posterior femoral condyle using 4 degrees external rotation. These two knees showed improved kinematics and horizontal joint line post- operatively.

Computer assisted navigation provides a precise understanding of the pre- operative knee kinematics. Bony cuts can be tailored to suit the pre- operative deformity. Increased external rotation of the femur with adequate medial soft tissue release is an alternate approach for difficult knees with ‘oblique joint lines’.

The ligament balancing technique involves precise measurement and equalisation of flexion and extension gaps. A force tension distractor that has separate arms for the medial and lateral joint compartments was used. We describe our experience of 40 total knee replacements (TKR) using this technique.

We undertook a prospective randomised trial using computer assisted navigation in TKRs applying two different soft tissue balancing techniques. The aim was to see how balancing techniques help us achieve a rectangular flexion extension gap. The 40 TKR that underwent the ligament balancing procedure were part of this trial. The distractor used was derived from the Freeman-Swanson knee instrumentation which measures the gap and tension in the medial and lateral compartments. The options to make the gap rectangular were: 1. adjustment of femoral cut by change in external rotation (for the flexion gap); 2. soft- tissue release or 3. a combination of both. Using computer assisted navigation it was possible to perform real time motion analysis during surgery.

We found that three degrees of external rotation for the femoral component was adhered to in only 16 out of 40 knees. The remaining 60% had external rotation of femoral component varying between two and eight degrees. No maltracking of the patella resulted in any of the TKR with increased rotation of the femoral component. The axis of movement was plotted on a graph at the end of the surgery by passive extension to flexion to which the operating surgeon was blinded.

Varying external rotation of femoral component might be an option in balancing difficult knees. Computer navigation enables precise tailoring of bony resection to suit different deformities.

Obesity [Body Mass Index (BMI) > 30kg/m2] is seen in a growing percentage of patients seeking joint replacement surgery. Operations in obese patients take longer and present certain technical difficulties. Computer navigation improves consistency of prosthetic component alignment but increases operation time.

Our aims were

to compare tourniquet times of non-obese with obese patients having knee replacement using standard instruments or computer navigation and

A retrospective analysis of 232 total knee replacement (TKR) operations performed by a single knee surgeon over a three year period was carried out. Similar knee prostheses (Plus Orthopedics, UK) were used in all cases. Variables to be assessed were the operative technique (computer navigation assisted or standard instruments) and BMI of patients.

Of the 232 knees, 117 were performed using computer navigation and 115 with standard instruments. Each of the groups was subdivided as per BMI to differentiate obese patients (BMI > 30) from the non-obese. Tourniquet times of surgery were used for comparison amongst the subgroups.

There were 56 and 59 patients in the non-obese and obese subgroups respectively within the standard TKR group. The average tourniquet times for these were 79.3 and 86.3 minutes respectively. This was a significant difference (p=0.037). Correspondingly in the computer navigated group, there were 60 non-obese and 57 obese patients. Their tourniquet times were 105.4 and 100.5 minutes respectively. This difference was not significant (p=0.15)

The obese patients in each group were then studied separately and divided into three equally sized subgroups in chronological order. Each sub-group comprised 19 standard TKRs and 19 computer navigated TKRs. Tourniquet times of operations were compared within each sub-group. P values within the first subgroup showed a significant difference. There was no significant difference within the second and third subgroups.

We concluded that obesity significantly increased the operative time in the standard TKR group. However in computer navigated TKR there was no significant difference in operative time between non-obese and obese patients. As the surgeon acquired experience of computer navigation there was no difference in time taken for conventional and computer navigated TKR in obese patients. We hypothesize that in obese patients, computer assisted navigation helps the surgeon to overcome jig alignment uncertainty without any time penalty.