Purpose: To determine whether subjective methods of assessing gaps and balance during total knee replacements are accurate.

Scope: 9 knees were included. Once the surgeon had balanced the knee using his standard subjective methods, an objective measure of the gaps and balance was taken using the Stryker knee balancer.

33% of knees were found to have unequal flexion extension gaps.
66% of knees were found to have imbalanced soft tissues.


Classical knee arthroplasty teaching states that when performing total knee replacements (TKRs) equal flexion, extension gaps should be achieved and equal soft tissue tension should be attained.1

In 1985, Insall stated that correct soft tissue balancing was the most important factor in determining the long term outcome following TKRs.1 Previous work, by the senior author has demonstrated the detrimental effect of imbalanced knee replacements.2

Various subjective measures are utilised by surgeons to assess flexion extension gaps and soft tissue tension, including the use of spacer blocks, finger tension technique, and the 'feel' of soft tissue tension on the medial and lateral sides.

There also objective measures of soft tissue balance and flexion extension gaps available, one commercially available product is the Stryker knee balancer.


Two specialist registrars; both of whom were experienced in the use of the balancer with the senior author, performed 9 TKRs, under the supervision of other experienced consultant knee surgeons, all of whom used subjective measure of gaps and soft tissue balance. When the surgeon was ready to implant the definitive component, an objective measure of the flexion extension gaps, and soft tissue tension was made using the Stryker Knee balancer.

Figure 1 - The balancer, Image courtesy of Stryker

The balancer (Figure 1) (Stryker UK), consists of two metallic plates that are inserted into the knee joint after the initial bony cuts on the tibia and femur have been made. The plates sit flat against the cut surfaces of the femur and tibia and on turning the 'handle' clockwise the integral rack-and-pinion tensing mechanism distracts the plates with the effect of tensioning the soft tissues. There is a central scale on the balancer which measures the degree of distraction in mm between the two plates, hence assessing the flexion and extension gaps. As the two plates are distracted, the upper plate is free to pivot centrally in the coronal plane, under the influence of any tension in the soft tissues and so demonstrates the degree of imbalance between the medial and lateral structures. Measurements are recorded in degrees on a scale from 0-12 on a 'platform/dial' attached to the upper plate. Any deviation to the medial side is defined as varus. Any deviation to the lateral side is defined as valgus.

In this study unequal gaps were defined as a difference of 2mm or more between the flexion extension gaps. Soft tissue imbalance was defined as greater than, or equal to 3 degrees of imbalance in flexion or extension.


The results for all 9 patients are shown in table 1. All TKRs were relatively 'simple', as they all had a pre-op varus deformity of 5-15 degrees and were being performed for primary osteoarthritis.

These results show that 3 of the 9 (33%) knees balanced subjectively had unequal flexion-extension gaps, and 6 of 9 (66%) had unequal soft tissue balance. Two of the knees ended up being imbalanced into valgus, due to the initial soft tissue release being too great.

Table 1
Patient age Releases performed Flexion gap Extension gap Difference in gaps Balance in flexion Balance in extension
1 78 Initial medial 24 24 0 0 0
2 93 Initial medial 24 24 0 9 Varus 12 Varus
3 61 Initial medial 24 28 4 3 Valgus 3 Valgus
4 73 Initial medial+limited deep MCL 34 34 0 3 Valgus 3 Varus
5 83 Initial medial 22 22 0 3 Varus 3 Varus
6 77 None 24 24 0 0 0
7 79 None 24 22 2 6 Varus 6 Varus
8 58 Initial medial 26 28 2 6 Varus 6 Varus
9 68 None 24 24 0 0 0
Initial medial release is defined as - Release of all soft tissues, upto, but not including the pes anserinus, and not including any release of either the deep or superficial MCL


In the 1970's Insall 3 and Freeman 4 were the first to emphasize the concept of soft tissue balancing and introduced the use of a spacer-tensor device in order to assess the symmetry of the flexion and extension gaps. They expressed the view that correct soft tissue balancing would increase the longevity and decrease revision rates in Total Knee Arthroplasty.

Sambatakakis et al,2 described the “cement wedge sign”: a smoothly tapering wedge of cement visible beneath the horizontal portion of the tibial component on anteroposterior radiographs. Evidence suggested this indicated persistent soft tissue imbalance after Total Knee Arthroplasty and was associated with a highly significant increase in radiolucent lines at the tibial bone-cement interface at follow-up.

Instability post-operatively is often caused by incorrect ligament balancing.5 The relationship between the adequacy of ligament releases and the severity of polyethylene wear, as found at revision surgery, was confirmed by Wasielewski et al.6 The soft tissue imbalance was associated with component loosening, polyethylene wear and overall failure of the Total Knee Arthroplasty.

In 1996, Attfield et al7 reported that knees balanced in both full extension and in flexion showed a significant improvement in proprioception post-operatively, whereas those balanced in extension only, did not.

Unitt et al8 have shown that, extensive soft tissue releases to obtain accurate objective balance significantly improve short-term clinical outcome without increasing complications and that balancing an imbalanced knee significantly improves short-term knee outcome.

Our results have shown that even in relatively 'simple' TKRs, subjective methods of assessing gaps and balance are inaccurate. They have also demonstrated that performing medial releases without an objective guide, can lead to the knee being imbalanced into valgus, due to too great an initial release.

A number of criticisms of the study do exist. Although both SpRs taking the measurements with the balancer were experienced in its use with the senior author, there is no torque meter on the balancer. Therefore 100% reliable measurements cannot be guaranteed, as it is possible different torques were produced in the measurement of the gaps. This flaw in the design of the balancer as a research tool is currently being worked on with the manufactuers.

A potential flaw in the surgical technique, using either subjective or objective techniques of balancing is

  1. Often the balancing is performed with the patellar everted, which may in itself affect the balance,
  2. The medial retinaculum is not closed at the time of assessment of balance, and hence may affect the accuracy.

In conclusion, if a surgeon wishes to achieve a TKR with equal flexion and extension gaps, and balanced ligaments then this study would suggest that they need to be guided by an objective rather than a subjective method.

Acknowledgements: None

External sources of support: None


  1. Insall JN (1985) Correction of arthritic deformities of the knee, In: McCarty DJ (ed) Arthritis and allied conditions. A textbook of rheumatology (10th ed). Lea and Febiger, Philadelphia pp. 771-784
  2. Sambatakakis A, Wilton TJ, Newton G. Radiographic sign of persistent soft-tissue imbalance after knee replacement. J Bone Joint Surg [Br] 1991; 73-B: 751-756.
  3. Insall JN (ed) (2000). Surgery of the Knee. Churchill Livingstone, New York pp. 1558-1562.
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  7. Attfield SF, Wilton TJ, Pratt DJ, Sambatakakis A. Soft Tissue Balance and Recovery of Proprioception after Total Knee Replacement. J Bone Joint Surg [Br] 1996; 78-B: 540-545.
  8. Unitt L, Sambatakakis A, Johnstone D, Briggs TWR and the balancer study group. Short term outcome in total knee replacement after soft tissue release and balancing. J Bone Joint Surg [Br] 2008; 90-B: 159-165.