ORIGINAL ARTICLE

Fixed flexion deformity after total knee arthoplasty is reduced by computer navigation

Martin P Laird*,George Kirsh*,Jacob Kaplan*

*Department of Orthopaedics Bankstown Hospital Bankstown, Sydney, NSW 2200 Australia

Address for Correspondence

Martin Laird
PO Box 32, Randwick, NSW 2031
Australia
phone : +61 431 050 945
Email : mlaird@gmp.usyd.edu.au

Abstract:

Introduction:
The benefit of computer navigation in total knee arthroplasty (TKA) remains controversial. Some studies have demonstrated a greater accuracy of component placement with navigation; however its clinical benefits remain unproven. Numerous studies have demonstrated a strong relationship between fixed flexion deformity and post-operative knee function.

Methods:

We undertook a partly retrospective, partly prospective study to compare post-operative flexion deformity in computer navigated versus conventional TKA. We examined 86 knees in 65 patients – 55 navigated and 31 non-navigated. All TKAs were performed by the same surgeon in one of two hospitals, changing from conventional instrumentation to navigation in early 2006. Fixed flexion deformity (FFD) was measured by blinded investigators with a goniometer an average of 3 years post op for non-navigated knees and 1 year for navigated knees.

Results:

Computer navigated knees had a statistically better FFD values than conventional TKR (0.7º vs 4.1º; p=0.007). There was no difference in flexion range (107º vs 104º; p=0.32). There was no difference in FFD for posterior stabilized (PS) vs cruciate retaining (CR) TKRs (2.1º vs 1.5º; p=0.64), though PS knees had greater range of flexion (108º vs 100º; p=0.008)

Clinical significance:

Our data suggests that computer navigation leads to less fixed flexion deformity after total knee arthroplasty. As FFD has been correlated with functional outcome, this suggests that navigation results in improved function after TKA.

J.Orthopaedics 2011;8(4)e3

Keywords:

TKA; computer navigation; fixed flexion deformity

Introduction:

The use of computer navigation has been gaining interest in orthopaedic surgery in recent years, particularly in regards to total knee arthroplasty (TKA). Debate still rages, however, as to whether computer navigation results in improved outcomes or greater longevity of the prosthesis. Proponents have argued that computer navigation leads to improvement in alignment, or a decrease in the number of outliers[1-6], whilst other authors have not found a difference between the two methods[7-10], and the evidence remains conflicting. To date, however, no study has specifically examined the role of computer navigation in decreasing fixed flexion deformity in TKA in vivo.
Fixed flexion deformity (FFD) is a common problem in knee arthritis. Inability to fully extend the knee disables the screw-home motion of the knee, leading to constant strain and early fatigue of the quadriceps when mobilizing, as well as increased forces through the patella and thus potentially earlier wear[11-13]. As such, correction of FFD at the time of arthroplasty is one of the more important challenges facing the surgeon. Various methods are employed to correct FFD, including a more proximal distal femoral cut, recession or resection of the PCL, removal of osteophytes and soft tissue releases especially of the posterior capsule. Most surgeons rely on their clinical judgement intraoperatively when assessing correction of FFD. It has been argued that computer navigation leads to improved component position and alignment, in both coronal and sagittal planes[1, 3, 4]. Navigation provides detailed information intraoperatively regarding these parameters. It has been shown in a cadaver study that assessing FFD via computer navigation is more reliable than clinical estimation by the surgeon[14]. Thus it is hypothesized that the use of navigation may decrease the incidence of FFD in TKA.

Material and Methods :

We examined 86 total knee replacements in 65 patients all performed by a single surgeon (GK) in one of two Sydney hospitals over a period of 6 years from 2001-2007. The surgeon changed from employing conventional technique to computer navigation in late 2005. Patients had either a Scorpio, Scorpio-Flex or NRG prosthesis (Stryker Orthopedics, Mahwah, NJ) inserted depending on the prosthesis used in the hospital at the time of surgery. The procedure was carried out via a midline incision with a medial parapatellar arthrotomy. Conventional arthroplasty was performed using an intramedullary guide for the femur and extramedullary for the tibia. Computer navigation was performed using the Stryker Knee Navigation System 3.0/3.1 via single pins inserted into the distal femur at the proximal end of the primary incision, and mid tibia via small stab incisions.

Patients were examined either as part of a routine follow up visit, or were contacted and asked to attend for examination purposes. Patients were approached via a list of sequential procedures done by the treating surgeon over this time. All patients provided informed consent, and prior ethics committee approval was obtained. Knees were primarily evaluated for extension limit or fixed flexion deformity, and a subset of patients were evaluated for satisfaction with their knee replacement via an analogue scale. Measurements of FFD were performed by blinded assessors (ML or JK) using a goniometer, with patients supine on an examination table, the patient’s ipsilateral foot supported on a pillow and the patient asked to extend the knee as far as possible. On-table computer navigation data were not used in the evaluation, as it was felt this would represent an artificial situation and be less relevant than clinically measured data taken at a reasonable period post operatively. Clinical review was performed at a minimum of 6 weeks postoperatively, and an average of 1.6y – (2.8y for the conventional group and 1.0y for the navigated).  

Results :

The 65 patients comprised 46 women and 19 men (27% men in navigated vs 32% men in conventional group). Average age was 73.0 (72.5 in navigated group vs 73.9 in conventional). Pre-operative FFD scores were only recorded in the notes on a subset of patients (19 in the conventional group, 12 in the navigated group). There was no significant difference between these subgroups (10.8º in the conventional vs 10.0º in the navigated group, p=0.82) in pre-operative FFD.

There was no statistically significant difference between the two groups in terms of type of prosthesis used (CR vs PS), age or sex as determined by Fisher’s exact test. More knees were cemented in the navigated group than the conventional group (37% vs 5%) as the senior surgeon tended towards cemented fixation later in the study, however it was not felt that the method of fixation would affect the degree of FFD.
The degree of FFD was compared between the two groups using the Student’s t-test. Computer navigated knees had statistically better fixed flexion deformity values than conventional TKA (0.7º vs 4.1º; p=0.007). There was no difference in range of flexion (107º vs 104º; p=0.32). There was no difference in FFD for posterior stabilized vs cruciate retaining TKAs (2.1º vs 1.5º; p=0.64), though PS knees had greater range of flexion (108º vs 100º; p=0.008).

In the subgroup of 41 knees evaluated for satisfaction using an analogue scale, navigated knees scored 8.6 vs 7.7 for conventional out of 10. This result was not found to be significant (p= 0.14). Patients with satisfaction scores greater than 8.5 were found to have significantly lower FFD than those with scores less than 8.5 (0.8º vs 4.5º, p=0.01).

These results are summarised in table 1.
Table 1

Discussion :

Fixed flexion deformity is a significant clinical problem. Inability to fully extend and thus lock the knee causes increased strain and fatigue of the quadriceps musculature both whilst walking and standing still, and increased patellofemoral contact pressures. Resultant anterior thigh and knee pain can lead to reduced function[11-13, 15]. Thus correction of FFD is one of the more important goals of the treating surgeon. It has been shown that computer navigation leads to better assessment of FFD than direct clinical evaluation in a cadaver study[14]. This result is likely to be further magnified on the operating table where anatomic landmarks are concealed by draping and tourniquets. Numerous authors have demonstrated an improvement in sagittal plane alignment with navigation[1, 3, 4]. However to our knowledge navigation has not previously been shown to reduce the incidence of FFD after TKA in vivo.
Our results demonstrate a small but statistically significant improvement in fixed flexion deformity with computer navigation vs conventional instrumentation, of approx 3.4º. It is difficult however to gauge the clinical significance of this difference. We could find no data correlating the exact degree of FFD with a measurable loss of function. The only surrogate measure of function used in our study – patient satisfaction as measured by analogue scale – was not significantly different between the navigated and conventional groups. However we did find that patients with higher satisfaction scores (>8.5) had significantly lower FFD (0.8º vs 4.5º) suggesting that even this small difference leads to clinical differentiation. It has been demonstrated elsewhere that surgeons tend to round estimates of FFD to the nearest 5º, suggesting that they feel this is the limit of accuracy that can reasonably be achieved with the naked eye [14]. This was also born out in our study, despite the use of goniometers to measure FFD . This suggests that it is likely to be difficult to prove that an improvement of 3.4º leads to a clinically important improvement in outcomes.

There were a number of limitations to our study. It was not a randomised study, so despite the absence of statistically significant differences between the two test groups it is possible that the groups were not completely similar prior to the intervention under study. Similarly, since the study was done in sequence, with most non-navigated knees done earlier than the navigated knees, it is possible that the surgeon’s technique changed in some other unrecognised way during the study. As with any study, the findings could be strengthened with the inclusion of greater numbers of patients.
In conclusion, this study suggests that computer navigation leads to a reduction in fixed flexion deformity after total knee arthroplasty, to a small but statistically significant degree (ie. 3.4º). This result lends support to the argument that computer navigation leads to improved functional outcomes and increased longevity of TKAs. However further studies are required to corroborate our findings and clarify the clinical importance of this result.

Reference :

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This is a peer reviewed paper 

Please cite as :Martin P Laird,Fixed flexion deformity after total knee arthoplasty is reduced by computer navigation .

J.Orthopaedics 2011;8(4)e3

URL: http://www.jortho.org/2011/8/4/e3