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Assessing The Accuracy Of Surgeon Estimation Of Range Of Movement Of The Knee During Total Knee Replacement

Leela C Biant*, Sebastian Herrmann*, Warwick JM Bruce#, Peter M Walker#, William R Walsh**

*Orthosports Clinic Sydney and Concord Repatriation General Hospital, Sydney,Australia
Surgical and Orthopaedic Research Laboratories, University of New South Wales, Sydney, Australia
# Orthosports Clinic Sydney and Concord Repatriation General Hospital, Sydney,Australia
** Surgical and Orthopaedic Research Laboratories, University of New South Wales, Sydney, Australia

Address for Correspondence:
Leela C Biant
Orthosports Clinic
47-49 Burwood Road
, Concord,Sydney NSW 2137
Fax    : +44 1932 852543
Phone: +44 7930 338569

Email :



Intra-operative estimation of range of flexion, extension, varus and valgus of the knee is important in ligament balancing and implant selection during total knee replacement (TKR). Many outcome studies of TKR are also based on surgeons’ estimation of post-operative range of movement. There are currently no published papers which demonstrate how precise surgeon assessment of range of movement of the knee is. This study evaluated the accuracy of surgeons’ estimation of maximal range of flexion, extension, varus and valgus of the knee compared to simultaneous computer navigated reading of the same knee movements during TKR in 100 patients. The surgeons were very accurate at assessing extension, varus and valgus but consistently underestimated flexion of the knee

J.Orthopaedics 2007;4(2)e21


Clinically assessing the range of motion and laxity of a knee intra-operatively is an essential part of total knee replacement. The surgeon relies on this judgement of flexion, extension, varus and valgus for implant selection and optimal ligament balance. Assessing the range of movement of the knee in the outpatient setting is also an outcome measurement for many studies of TKR1-3. However, to our knowledge there are no published studies which evaluate how accurate surgeons are at estimating knee range of movement intra-operatively. This study evaluated the surgeons’ assessment of maximal range of knee flexion, extension, varus and valgus in flexion and varus and valgus in extension compared to simultaneous computer navigation reading of the same knee movement.

Material and Methods :

100 consecutive patients undergoing primary posterior stabilized Genesis II TKR (Smith and Nephew) with Brainlab image free computer navigation (Brainlab Australia Pty Ltd) were studied. There were no exclusion criteria for patients in this study. Four surgeons participated, two experienced knee arthroplasty surgeons and two senior orthopaedic fellows.

A thigh tourniquet was used for all patients. The tibial and femoral pins for the fixed navigation points were inserted. The knee was accessed via a medial parapatellar approach, and the acquisition of mapping points for the navigation system completed. Accuracy of the pin placement is not relevant as the computer builds a picture of the mechanical axis in 3D based on over 200 acquisition points provided by the surgeon, including the malleoli, Whiteside’s line, the antero-posterior tibial direction and the hip centre. These points are referenced to the pins in place, not the pin placement providing the reference. Bone resections were performed with guidance from the Brainlab image-free navigation system.

During the final trial reduction of implants, a towel clip was applied to the patella retinaculum to simulate knee closure. The surgeon estimated the range of maximal knee flexion by holding the thigh vertically with the hip flexed 90 degrees and allowing the knee to flex freely under gravity (figure 1), the surgeon used a goniometer to measure the maximal flexion. For the surgeon to evaluate extension of the knee, the patients’ foot was supported and the knee allowed to extend freely to its’ maximal extension position under gravity (figure 2). Varus and valgus of the knee were evaluated with the knee in extension, and then with the knee in 90 degrees of flexion.

Figure 1. Position of the leg for evaluation of knee flexion

Figure 2. Position of the leg for evaluation of knee extension

The surgeon was unable to see the computer during the stage of the operation where the range of movement was evaluated. The estimation of range of movement was compared to that displayed simultaneously by the computer navigation system. The difference in degrees of angulation between the surgeon estimation of a movement in one plane and the computer measurement of the same movement was calculated.

Results :

The overall results are summarized in table 1. The surgeons consistently underestimated knee flexion, the mean underestimation of knee flexion was 12.8° for each patient. The surgeons were good at true estimation of varus and valgus compared to the computer navigation measurement of the same angles, showing accuracy to within one degree. Surgeon estimation of true extension of the knee was also accurate, any discrepancy from the true value was an overestimation of any hyperextension

Movement of the knee Mean difference between surgeon estimation and computer measurement in degrees of angulation (standard deviation)
Extension 0.10           (3.8)
Valgus in extension 0.11           (0.96)
Varus in extension 0.02           (1.17)
Flexion 12.8           (6.4)
Valgus in flexion 0.06           (2.28)
Varus in flexion 0.29           (1.69)


There was no inter-observer difference between the individual surgeons estimation of range of movement. All four surgeons consistently underestimated flexion to the same level of inaccuracy. The two most experienced surgeons were marginally better at estimating varus and valgus, but because the degree of inaccuracy was so low this was not significant

Discussion :

Surgeons consistently underestimated intra-operative knee flexion compared to simultaneous computer measurement of the same knee position. They were reasonably accurate at evaluating knee extension, varus and valgus.

There is no consensus on how knee flexion should be measured. Some surgeons may take the angle subtended by the anterior contour of the knee. Others use the estimation of the centre of the knee as the fulcrum and align the goniometer along the line of the skeleton from this point. The navigation system measures flexion from its calculation of the mechanical axis of the leg, this means that the fulcrum (point of maximal angulation) is actually anterior to the knee in full flexion.

It may be difficult for the surgeon to accurately estimate knee flexion intra-operatively in the obese patient, the line of the skeleton and hence the mechanical axis is not easy to view.

Surgeons are good at estimating varus and valgus. This is of utmost importance in balancing the knee and selection of a tibial polyethylene insert size. Clinically it is important that the surgeon can accurately evaluate whether a knee can fully extend or if hyperextension is occurring. Our study showed that the surgeons were good at this. 

One limitation of this study is that only four surgeons participated from a high volume arthroplasty unit, it may be that these surgeons were not a representative sample of surgeons performing total knee arthroplasty. Also, the majority of TKRs performed by the surgeons in the months prior to the study were also computer navigated. It may be that during the learning curve and subsequent experience with the computer navigation a surgeon gets position feedback which improves judgement of flexion, extension, varus and valgus.

Clinical implications of underestimation of knee flexion in TKR are not thought to be significant for the individual patient, but may be of importance in follow-up studies of different TKR prostheses.

Reference :

  1. Hewitt B, Shakespeare D. Flexion vs. extension: a comparison of post-operative total knee arthroplasty mobilisation regimes. Knee 2001 Dec;8(4):305-9

  2. Pope RO, Corcoran S, McCaul K, Howie DW. Continuous passive motion after primary total knee arthroplasty. Does it offer any benefits? J Bone Joint Surg Br1997 Nov;79(6):914-7

  3. Harvey IA, Barry K, Kirby SP, Johnson R, Elloy MA. Factors affecting the range of movement of total knee arthroplasty. J Bone Joint Surg Br 1993 Nov;75(6):950-5


This is a peer reviewed paper 

Please cite as :Leela C Biant : Assessing The Accuracy Of Surgeon Estimation Of Range Of Movement Of The Knee During Total Knee Replacement

J.Orthopaedics 2007;4(2)e21





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