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Knee: J. M. Spencer, S. K. Chauhan, K. Sloan, A. Taylor, and R. J. Beaver Computer navigation versus conventional total knee replacement: NO DIFFERENCE IN FUNCTIONAL RESULTS AT TWO YEARS J Bone Joint Surg Br 2007; 89-B: 477-480 [Abstract] [Full text] [PDF]

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Author's reply:

Richard J Beaver   (25 June 2007)

CT proves that TKA needs a tibial slope

Justin P Cobb   (17 May 2007)

Author's reply: 25 June 2007
Richard J Beaver, Orthopaedic Surgeon Royal Perth Hospital Send letter to journal: Re: Author's reply: rjbeav{at}bigpond.com Richard J Beaver	Sir, We thank Professor Cobb for his interest in our paper. The original study randomly allocated patients to one of two groups: navigated or non-navigated. The alignment differences between these groups were documented. The only significant differences between the two groups related to implant position. This present paper objectively assesses the mid-term functional results of the two groups. We are therefore assessing two groups of patients with different component alignments. We too believe that accuracy matters but we can only report our mid-term results in as unbiased a manner as possible. It may well be that other studies will obtain differing functional results. We would be delighted to be able to prove navigation a significant advance but the last thing we would wish is to publish opinion which is not supported by evidence. It may be that the functional and survivorship differences between the two groups will only surface at the five or ten year mark (which we also hope to publish). We are also preparing for publication the results of our institution’s entire experience in knee navigation (350 cases), involving a number of surgeons with varying degrees of skill and comparing these to our non-navigated patients. It may well be that the real place of navigation is in improving long-term survivorship or in reducing peri-operative morbidity. R.J. BEAVER, FRACS, Orthopaedic Surgeon, Royal Perth Hospital, Perth, Western Australia.
CT proves that TKA needs a tibial slope 17 May 2007
Justin P Cobb, Orthopaedic Surgeon Imperial College Send letter to journal: Re: CT proves that TKA needs a tibial slope j.cobb{at}imperial.ac.uk Justin P Cobb	Sir, While I do not support the use of ‘image free’ navigation systems in general, in their title1 the authors make an unjustified statement that is in danger of being taken out of context, and setting back the cause of accurate surgery. They are to be congratulated on having carried through an important study, attempting to correlate accuracy and function, but the title and conclusions of this paper are not supported by their data, which compares two groups of total knee replacements which were inserted in substantially different positions. In the 2004 paper which is followed up in the current paper, the two groups of knee replacements were inserted in different positions. The conventionally inserted group’s tibial components were inserted with a substantial posterior slope, mode 10°, while the navigated group's tibial components were inserted with little or no posterior slope, mode 2° (p<0.001). This difference is a significant change in surgical technique. The Duracon knee was designed for insertion with a posterior slope, but in this series, even if inadvertently, the surgeons used the navigation to put the prosthesis in a substantially different inclination to the conventional group. The conventionally inserted group, while more variably inserted, were actually inserted in the main rather well, by a surgeon who was clearly an expert. This earlier paper could thus be seen as an experiment, albeit one that was not planned, with the post-operative CT scans confirming the difference between the two groups who could thus be renamed: posterior slope/conventional and minimal posterior slope/navigated. In this clinical follow-up study, the knee scores of the minimal posterior slope/navigated group were not better, but actually worse (mean Oxford scores of 26 vs 20), although this difference fails to reach significance. If the slope of the tibia was used in a univariate analysis, the authors might instead republish this series as conclusively showing that an increased posterior slope of the tibia is an important determinant of outcome following this particular total knee arthroplasty. A multivariate analysis of outcome in this important cohort would confirm whether this was the most important variable. Total knee arthroplasty is an expensive and commonly performed operation, with a significant failure rate predicted,2 and a substantial rate of dissatisfaction if judged by scores designed to discriminate.3,4 Surgeons should not take the wrong message away from this series of operations by an opinion leader in total knee arthroplasty. Accuracy really does matter. We are still learning how best to perform this operation, and this paper suggests that a greater posterior slope is one factor that helps the function of this device, although variation between individuals may be substantial. If a knee is not functioning very well, it may be that a mismatch with the patient's native posterior slope is one of the problems. As the authors have shown in their other work, a CT scan may help greatly in the analysis, even if a lower dose is now practicable.5 Alternatively, by performing the CT scan beforehand to document the pre-operative position and plan what is to be corrected, surgeons may one day choose to tailor the operation to each individual patient. This seems to work.6 J.P. COBB, MCh, FRCS, Orthopaedic Surgeon, Imperial College London, London, UK. 1. Spencer JM, Chauhan SK, Sloan K, Taylor A, Beaver RJ. Computer navigation versus conventional total knee replacement: no difference in functional results at two years. J Bone Joint Surg [Br] 2007;89-B:477-80. 2. Kurtz S, Ong K, Lau E, Mowat F, Halpern M. Projections of primary and revision hip and knee arthroplasty in the United States from 2005 to 2030. J Bone Joint Surg [Am] 2007;89-A:780-85. 3. Weiss JM, Noble PC, Conditt MA, et al. What functional activities are important to patients with knee replacements? Clin Orthop Relat Res 2002;404:172-88. 4. National Joint Registry for England and Wales, 2nd Annual Report, September 2005. http://www.njrcentre.org.uk/documents/reports/annual/2nd/NJR2_fullreport.pdf (accessed 17/05/07). 5. Henckel J, Richards R, Lozhkin K, et al. Very low-dose computed tomography for planning and outcome measurement in knee replacement. The imperial knee protocol. J Bone Joint Surg [Br] 2006;88-B:1513-8. 6. Cobb J, Henckel J, Gomes P, et al. Hands-on robotic unicompartmental knee replacement: a prospective, randomised controlled study of the acrobot system. J Bone Joint Surg [Br] 2006;88-B:188-97.