Authors: Ayaz Lakdawala MRCS, MBBS - SpR - Trauma & Orthopaedics, Royal Orthopaedic Hospital, Birmingham
Nick Rouholamin MRCS - SpR - Trauma & Orthopaedics, Russells Hall Hospital, Dudley
Nadim Aslam FRCS (Orth) - Consultant Orthopaedic Surgeon - Worcestershire Royal Hospital, Worcester
Component alignment and ligament balancing are critical factors in achieving a successful functional outcome following TKA1. Malalignment is an important cause of early failure. This can cause pain, instability, reduced range of movement, excessive polyethylene wear, and subsequent implant loosening. Conventional instrumentation uses anatomical bony landmarks. Reference errors with these landmarks can occur because these are either invisible (e.g., femoral head), virtual (e.g., mechanical axis) and difficult in the presence of associated deformities. Computer-assisted surgery (CAS) technology allows intra-operative quantitative measurements of axes. This information assists the surgeon in achieving component alignment and improved balancing of the knee, thereby avoiding the “outliers” in the alignment of the mechanical axis.
CAS: The Technology
Overall there are two mainstream technologies in current use: image based and imageless CAS. The former usually uses fluoroscopic assistance and has the ability to create a spatial link between the image and anatomical landmarks, the defined virtual points, planes, and axes. This enables the surgeon to visualise the fluoroscopic image of the implants intra-operatively and has control of every step in the procedure. Thus, with an image-based system, the surgeon can define the landmarks kinematically as well as visually. However, this has certain drawbacks. The fluoroscope is bulky, needs larger operating field & there is a potential radiation hazard. The imageless CAS is cheaper, less bulky, and easier to use.
In CT-based systems, scans are obtained pre-op and, intra-operatively the surgical field is registered and defined using either a surface-based or point-based system. Surface-based registration can provide a virtual 3D image and also provides assessment of bone density. This technique uses more radiation and can be costly.
The alternative is an image-free navigation. In this technique, the key anatomical references points (centre of the hip and ankle) are digitised by the surgeon. Accuracy is user dependent & complete 3D images cannot be obtained.
‘Bone Morphing’ is a newer technique and it provides complete 3D images. The computer computes the points digitised on the articular surfaces of the tibia and/or femur. Here the registration is done intra-operatively between the anatomical data and the statistical model. It cannot provide information on bone-density.
Virtual fluoroscopy is a system which does not require a registration procedure. The principle is to navigate on calibrated fluoroscopy images. After two to three images are acquired, the C-arm is removed. The images are 2-dimensional and expose the staff to radiation.
For the femoral component, the CAS technology provides the surgeon with precise information on flexion/extension and varus/valgus. For the tibial component, it provides more precise information regarding the orientation of tibial slope and varus/valgus positioning. This can assist in achieving more precise alignment and flexion/extension balancing of the knee.
Computer Assisted TKR Vs Conventional TKR
Different authors have published their experiences with these systems. A recent meta-analysis has shown more accurate AP & lateral alignment of the tibial and femoral components with fewer outliers outside the range of 3° varus or valgus 3. Some studies have shown no clinically significant difference between CAS & conventional TKR 4-6.
Sikorski 7 highlighted the limitations of the CAS to help identify rotational malalignment. Correct rotational alignment of a TKA is important to achieve optimal patellar tracking and implant longevity. In a randomised study, Lutzner et al 8; did not find any notable difference between CAS and conventional TKR techniques with regards to rotational alignment of the femoral or tibial components.
There is a steep learning curve, not only for the surgeon but also for the nursing staff. The operating time with CAS is longer than conventional technique because of the set-up and data acquisition 2. Most studies show not difference in blood-loss between the CAS and conventional technique. Functional results are also similar at 2 yrs 9. At present there is no long-term follow-up available on TKR using CAS technology.
BrainLAB’s knee essential software
CAS in Revision TKR
Common cause of revision TKR is aseptic loosening of the tibial components and instability due to inadequate balancing of the flexion/ extension gaps. In revision surgery the challenge is restoring the joint line and stability. This can be complicated by associated bone loss and difficulty in identifying relevant bony landmarks. CAS can help aid alignment and restore the joint line, however further information on CAS in revision TKR is required in the literature.
Different CAS imaging modalities are emerging. Recently bone morphing has been introduced, allowing visualisation of a 3D surface of the bone intra-operatively. Accuracy of these new technologies needs to be clinically validated and devices need to be regulated. What is needed is a CAS system that does not require pre-operative imaging, allows 3D reconstruction, uses percutaneous techniques of registration and is easy to use. It would be desirable to have a system that also facilitates minimally invasive TKR.
CAS is still evolving and new technologies are emerging. It has a definite advantage in achieving precise component alignment but there is a learning curve. It can be particularly useful in planning TKR in presence of deformities around the knee.
It remains to be seen whether long-term results of TKR can be improved using CAS technology. It is possible that in the future, CAS will be more widely used especially as the new generation of trainees will most likely “grow up” with this concept and practice.
1. Insall JN. Surgery of the knee. 2nd Edition . New York: Churchill Livingstone, 1993.
2. Victor, J. Computer assisted surgery: Coronal and sagittal alignment. In Total Knee Arthroplasty: A Guide to Get Better Performance, edited by J. Bellemans, M. D.Ries, J.Victor, Springer, New York.
3. Bauwens K, Matthes G, Wich M, et al. Navigated total knee replacement: a meta-analysis. J Bone Joint Surg [Am] 2007; 89-A:261-9
4. Ensini A, Catani F, Leardini A, et al. Alignments and clinical results in conventional and navigated total knee arthroplasty. Clin Orthop 2007; 457:156-62.
5. Stulberg SD, Yaffe MA, Koo SS. Computer- assisted surgery versus manual total knee arthroplasty: a case controlled study. J Bone Joint Surg [Am] 2006; 88-A (Suppl 4): 47-54.
6. Kim YH, Kim JS, Yoon SH. Alignment and orientation of the components in total knee replacements with and without navigation support: a prospective, randomised study. J Bone Joint Surg [Br] 2007; 89-B:471-6.
7. Sikorski, J. M. Computer assisted surgery and rotational alignment of total knee arthroplasty. In Total Knee Arthroplasty: A Guide to Get Better Performance, chap. 40, edited by J. Bellemans, M. D. Ries, J. Victor, Springer, New York, 2005.
8. Lutzner J, Krummenauer F, Wolf C, Gunther K.P., Kirshner S. Computer-assisted and conventional total knee replacement. A comparative, prospective, randomised study with radiological and CT evaluation. J Bone Joint Surg [Br] 2008;90-B:1039-44.
9. Spencer JM, Chauhan SK, Sloan K, Taylor A, Beaver RJ. Computer- navigation versus conventional total knee replacement: no difference in the functional results at two years. J Bone Joint Surg [Br] 2007; 89-B: 477-80.
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