39Computer-Assisted Surgery: Coronal and Sagittal Alignment 39 39

39 Computer-Assisted Surgery:

Coronal and Sagittal Alignment

J. Victor

Summary

Computer-assisted total knee surgery has spread rapidly in the orthopedic community over the past 5 years. This technology has the potential to position the components of the total knee arthroplasty exactly in the desired posi- tion, hence avoiding outliers in postoperative alignment.

This paper describes the currently available systems and their respective advantages and flaws. Accuracy and dif- ferential clinical outcomes are discussed, based upon the available literature.

Introduction

Today, total knee arthroplasty is considered a safe, reli- able, and predictable procedure, allowing patients with knee arthritis to maintain their activities of daily life and moderate sports activities. Correct positioning of the components is a key factor in this success. Component malposition can cause pain [10], limited range of motion [5], instability [16], polyethylene wear, and loosening of the implant [7, 23]. In the earlier days of total knee arthro- plasty,much attention was given to the correction of limb alignment, as the detrimental effects of remaining malalignment were obvious and well documented, both clinically [1, 2, 7, 9, 12, 16, 17, 18, 21, 26, 29] and biomechan- ically [3, 8, 11].

Over the past decades, the characteristics of normal limb alignment were described [20] and with the advent of well-designed instrumentation systems, more repro- ducible results were obtainable.Also, the increased surgi- cal training led to the perception among orthopedic sur- geons that these problems of postoperative malalignment were behind us. However, recent literature proves the op- posite [6,10,13,23]. A round table and multicenter evalua- tion of the French orthopedic community concluded that 31% of patients with major pre-operative coronal malalignment displayed a deviation of the mechanical axis of more than 5° postoperatively.

Jeffery et al. [13] noted good postoperative coronal alignment (mechanical axis deviation of less than 3°) in two thirds of their total knee arthroplasties. In one third

of the operated knees, mechanical axis deviation in the coronal plane was found. These knees had a mechanical loosening rate of 24% at 8 years, as opposed to 3% me- chanical loosening for normally aligned knees.

Sharkey et al. [23] showed recently that malalignment and malposition of components still play a significant role in the failure mechanism of modern knee prostheses.

The increase of computer processor speed and tech- nical evolution led to the development of computer as- sistance for orthopedic surgery (CAS). Many different systems were developed. Two mainstream technologies have prevailed in total knee surgery over the past 5 years: imageless CAS and image-based CAS, the latter being most often combined with fluoroscopy in total knee surgery. The typical characteristic of image-based CAS is that the system can create the spatial link be- tween the image and the anatomical landmarks, the defined virtual points, planes, and axes. This additional information in the form of a fluoroscopic image allows the surgeon to double-check the information relating to the important reference planes and axes. However, these systems do have a significant disadvantage: The image intensifier is a tool that is normally not present in the operating theater during knee arthroplasty. It is bulky, needs to be draped, and poses a potential risk for cont- amination of the wound. It also increases surgical time and carries a potential radiation hazard. The imageless systems do not have these disadvantages; they are cheaper, easier to use, less bulky, and create no radiation hazard.

The aura of technological supremacy was, from the early beginning, inherently associated with CAS and many orthopedic surgeons expected, without critical analysis, a better positioning of the components, leading to fewer outliers in postoperative alignment. A hype was born, and through press releases the potential patients were informed of the supremacy of CAS.

In contrast, very few scientific papers were published

in peer-reviewed journals. Even worse, the early studies

on imageless systems did not show improved coronal

alignment with the use of CAS [19, 22]. Some studies

compared the computer-assisted TKAs with a matched

control group [14] and were able to present better results

use of CAS in TKA but had no control group [25].

Two recent prospective randomized trials that com- pared CAS with manual instrumentation in TKA report- ed excellent results for the CAS group with very few [24]

or no [27] outliers. In the latter study we assessed the accuracy of the calculation of the kinematic center of rotation of the hip and compared the outcome between the patients that underwent TKA with and without image-based computer assistance.

Materials and Methods

A total of 100 patients were included in our study. No ex- clusion criteria regarding the amount of pre-operative malalignment, previous surgery, or primary diagnosis were applied.

Randomization was performed in permutation blocks of four.As such,the study was performed on a con- secutive series of primary TKAs, where every operating day included two conventional and two CAS cases.

Pre-operative assessment included International Knee Society (IKS) knee and function score, measurement of range of motion (ROM),and radiological examination,in- cluding digital standing full-leg films (

), antero- posterior (AP), lateral, skyline, and condylar radiographs (Philips easyvision 4.2). The radiology technicians were instructed on how to align the limb with the patella point- ing forward and the knee in maximum extension in order to obtain standardized full-leg radiographs.

In the conventional group, extramedullary align- ment jigs were used on the tibia and intramedullary jigs on the femur. The full-leg standing films were used for

(FAA) was calculated and used on the distal femoral cut- ting jig.

In the CAS group, a fluoroscopy-based spatial navi- gation device, using active and passive reference frames (hardware: iON,Medtronic SNT,Louisville,CO; software:

FluoroKnee, Smith & Nephew, Memphis, TN, and Medtronic SNT, Louisville, CO), was employed. This sys- tem includes the use of a fluoroscopic C-arm with a cal- ibration frame to allow for positional calibration of the fluoroscopic image. The calibration frame carries light- emitting diodes to allow for tracking by the system’s camera. In order to obtain ideal stability for the kine- matic determination of the center of the hip, the patient was stabilized on the operating table using two padded posts, positioned against the iliac crests. No reference frame was attached to the iliac crest. Dual, bicortical pin fixation was chosen to eliminate the potential of insuffi- cient rotational stability of the reference frame. Place- ment of the pins was percutaneous, after a small stab wound was made. At the beginning of the procedure a calibration shot was made with the C-arm and all in- struments were validated. The fluoroscopic images were acquired (AP and three quarters of the hip, AP and lat- eral of the knee,AP and lateral of the ankle) and labeled.

The limb was then moved in a circular fashion to obtain exact definition of the spherical motion that was de- scribed by the femoral reference frame. The computer calculated the kinematic center of rotation and posi- tioned this calculated center on the already obtained fluoroscopic image of the hip.

Hemostasis, wound closure, and rehabilitation proto- col were identical for both groups. At 6 weeks, the pa- tients were seen in the clinic and standard digital radi- ographs (AP, lateral, skyline) and digital full-leg standing X-rays were obtained.All standard X-rays were made us- ing fluoroscopic control to achieve correct orientation in the lateral and coronal plane. In those patients who had not yet achieved full extension at 6 weeks, the full-leg standing X-ray was taken at 3 months postoperatively. At this stage, IKS scores and ROM measurement were done for all patients.

On the fluoroscopic images of the hip with the su- perimposed calculated kinematic center of rotation that were obtained during surgery, the distance be- tween the anatomical center of the hip and the calcu- lated center of rotation was measured for every patient (see Fig. 39-1). Angles of femoral and tibial component position were noted on AP and lateral X-rays. The ref- erence on the ateral radiograph was the posterior tibial cortex for the tibial component and the posterior femoral cortex for the femoral component. Overall mechanical coronal alignment was measured on the full-leg standing X-rays.

39

⊡ Fig. 39-1. Kinematic center of the hip as calculated from mathemati- cal algorithm (center of small circle). The crossing of the two black lines represents the anatomical center of the hip

Results

Three patients were excluded from the study: One patient in the CAS group died of unrelated causes, one patient in the CAS group was changed to conventional instrumen- tation because of a lead failure intraoperatively, and one patient in the conventional group suffered an ipsilateral hip fracture during the postoperative period. No patients were lost to follow-up. The pre-operative data are sum- marized in

.

The intraoperative coronal alignment values, as com- puted during surgery by the CAS system before any bone cuts were made, and the measured pre-operative coronal alignments on the full-leg standing films were plotted against each other and are displayed in

.

The mean distance between the computed kinematic center of rotation and the anatomical center of the femoral head was 1.6 mm (range 0-5 mm, standard devi- ation 1.46). The mean angular mistake on the femoral mechanical axis was 0.19° (range 0°-0.71°).

The tourniquet time was longer in the CAS group than in the conventional group (72 min versus 60 min, p<0.0005), as was the skin-to-skin time: 93 min versus 73 min (p<0.0001).The mechanical axis in the coronal plane was divided into three groups: 0°-2°, 3°-4°, and >4°. In the CAS group all knees (n=49) scored within 0°-2°. In the conventional group (n=48), 73.5% of the knees fell with- in 0°-2° and 26.5% within 3°-4°. No knees displayed a mechanical axis deviation of more than 4° (

).

The difference in variance between the CAS group and the conventional group is significant: p<0.0001 (Fisher’s exact test). This means that the postoperative alignment tends to be more variable in the conventional group than in the CAS group.

No significant differences were noted between the CAS patients and the conventional patients concerning blood loss,lateral femoral angle,lateral tibial angle,patel- lar shift,patellar tilt,knee score,function score,and ROM.

IKS Knee score at 3 months postoperatively was 86 in the conventional group versus 87 in the CAS group. IKS func- tion score was 68 at 3 months postoperatively in both groups.

Mean flexion at 3 months was 116° (100°-125°) in the conventional group versus 114° (90°-125°) in the CAS group. Fixed flexion contracture was present pre-oper- atively to an extent of :10° in four patients, 5°-9° in 49 patients, and <5° in 44 patients. Postoperatively, this was reduced in the conventional group to one patient, seven patients, and 41 patients, respectively, and in the CAS group to 0 patients, four patients, and 44 patients, respectively.

Complications in the conventional group included de- layed wound healing (3),bleeding requiring evacuation of

Full Leg Measurement C

A S M e a s u r e m e n t

⊡ Fig. 39-2. Plot of the pre-operative coronal alignment measurements on full-leg standing radiographs (X-axis) versus the intraoperative coronal alignment measurement by the image-based CAS system (Y-axis). Positive angular values represent mechanical axis varus alignment, negative an- gular values represent mechanical axis valgus alignment. Pearson corre- lation index r=0.987

⊡ Table 39-1.Demographic data and pre-operative distribution of variables

CAS Conventional

Mean Range Mean Range

Age (years) 72 56-85 70 40-83

Weight (kg) 76 50-102 78 52-92

Length (m) 1.64 150-176 1.64 149-178

Male/female 13/37

Varus/valgus 29/21 32/18

Deformity (mech. axis) 8.4 0-16 7.5 0-19

Flexion 105 80-120 111 90-125

Ext. deficit 16 10-20 17 10-20

PS/CR 18/30 23/26

hematoma (1), and symptomatic deep venous thrombo- sis (1). Complications in the CAS group were delayed wound healing (1), pin breakage (3), symptomatic deep venous thrombosis (1), urological (1), and reflex sympto- matic dystrophy, successfully treated with epidural sym- pathetic block (1). Mean fluoroscopy time was 21 s (9-34 s), corresponding to a mean radiation of 0.19 Gy/cm

.

Discussion

Malalignment in the coronal plane is still an important cause of early failure of TKA, despite improved surgical training and instrumentation [6,13,23].The cause of post- operative malalignment is most often incorrect bone cuts.

The error in performing the bone cut can be categorized as a ‘reference error’ or an ‘execution error’. Convention- al instrumentation uses bony references (anatomical landmarks) or ligament references.Reference errors with

anatomical landmarks occur because the references are not visible (as with the femoral head), are virtual in na- ture (as with the femoral mechanical axis),or are variable (dysplasia, bowing of long bones, morphotype variabili- ty) (

).

The recent popularity of less invasive exposures for ZKA will certainly not reduce the incidence of reference errors. Ligament referencing is not an alternative for this problem and will not preclude these mistakes, as the tibial bone cut is used as a base reference and as the ligaments as such may be shortened or damaged during surgery.

One might assume that the use of computer assistance would rule out surgical errors,but this needs to be proven in controlled trials. The total error in CAS TKA will con- sist of the sum of the ‘camera error’, the ‘algorithm error’, and the ‘execution error’. The latter will be similar to the situation with conventional instrumentation and is caused by displacement of the blocks during pinning or saw blade inaccuracy. The advantage of CAS is that block displacement can be detected prior to cutting and that the bone cut can be checked afterwards.

The ‘camera error’ and ‘algorithm error’ are specific for the hard- and software used and were the subject of this study. The first aim was to assess the accuracy of the kinematic determination of the center of rotation of the hip. Many navigation systems are employed in current orthopedic practice today, and most of them use a math- ematical algorithm to define this center. Several in vitro tests have been carried out, mainly by the manufacturers but little has been published about the clinical perfor- mance [27]

The setup of this study allowed us to do this, as both kinematic and spatially linked radiographic data were available.The accuracy was excellent.The mean deviation between kinematic and radiographic center was 1.6 mm (range 0-5 mm).As stated before, this result is specific for the given hardware and software that was used. The soft- ware was very sensitive to pelvic motion, and the pelvis

39

40 35 30 25 20 15 10 5

0 Vl >=10 VL 9-5 VL 4-3 2-0-2 VR 3-4 VR 5-9 VR >=10

18 10 3 13 5 32 16

0 0 7 36 6 0 0

0 0 0 48 0 0 0

Pre-operative Conventional CAS

⊡ Fig. 39-3. Distribution of pre-op- erative, conventional postoperative, and CAS postoperative mechanical alignment, as measured on full-leg standing films

⊡ Fig. 39-4. Dysplastic nature of the proximal tibia with a largely ab- normal slope of the proximal tibia.

Using the ‘natural’configuration of the tibial surface could lead to a major malalignment in the sagittal plane

needed to be stabilized on the operating table with padded posts. It might be that algorithms less sensitive to pelvic motion are less accurate. With image-based systems, intraoperative control is possible. This is not the case with imageless systems, where the orthopedic surgeon has to rely on the information provided by the manufacturer, and accuracy can be deduced only in- directly in evaluating the postoperative alignment.

The second arm of the described study was a differ- ential outcome analysis between the two groups.The lim- its of this study lie in the use of full-leg standing radi- ographs as an outcome tool for measuring alignment.

Full-leg radiographs are far more reliable for measuring coronal alignment than the 14 × 17-inch films that are of- ten used for this purpose [15,25],the reason being that the angle between the femoral mechanical and femoral anatomical axis is variable [28]. One needs to see the femoral head before the femoral mechanical axis can be accurately determined. Full-leg radiographs have been used in previous studies [22] without correlation between the pre-operative measured values and the intraoperative computed values.

We were able to validate the use of full-leg standing ra- diographs for measuring coronal alignment in this study.

The measured value on the film was compared with the computed intraoperative value,before the bone cuts were made. The figures matched exactly (1° or less) for 82%

of the knees, as witnessed by the correlation index (see Fig. 39-2). Only two knees had a mismatch of more than 3° between the measured pre-operative value and the calculated intraoperative value.These patients had severe pre-operative fixed flexion contracture (>15°), explaining the inaccuracy of the pre-operative full-leg standing radiograph. As these fixed flexion contractures were corrected after surgery, one can conclude that the post- operative full-leg radiographs were reliable for measuring postoperative coronal alignment.

Our results are excellent in the CAS group: All knees displayed between 0° and 2° of mechanical alignment.

This fact is paramount, as it is the main justification for the use of CAS in TKA. Previous studies did not report equally good results.Saragaglia et al.[22] took a wider er- ror margin (0°-3°) and scored within this margin for 84%

of the knees that were operated on with computer assis- tance (Orthopilot, Braun Aesculap, Tuttlingen). Two of 25 knees were ‘big’ outliers: 5° varus and 7° valgus in the mechanical axis.In the conventional group the results are comparable to ours: They reported 75% of the knees with- in the 0°-3° margin, versus 73.5% within 0°-2° for our series.Mielke et al.[19] compared 30 navigated cases (Or- thopilot) with 30 matched patients who were treated con- ventionally. In the CAS group they reported 61.7% within 0°-2°; 6.7 % in this group had a deviation of more than 4°.

Two knees were ‘big’ outliers: 5° and 7° varus. In the con- ventional group, 10% of the knees displayed a deviation

of more than 4°. Jenny and Boeri [14] compared 40 navi- gated cases (Orthopilot) with 40 matched conventional cases. The error margin was defined between 0° and 3°;

95% of the CAS knees scored within this range, versus 85% of the conventional knees.Bäthis et al.[4] performed a randomized trial of 160 patients with the Brainlab Vec- tor Vision system and achieved good mechanical align- ment in the coronal plane (0°-3°) for 97% of the patients in the CAS group versus 74% in the conventional group.

Sparmann et al. [24] studied the outcome of using an imageless navigation system (Stryker Howmedica Os- teonics, Allendale, NJ) prospectively on a large cohort of 120 navigated cases versus a control group of 120 conven- tional cases: 97.5% (117/120) in the CAS group displayed a mechanical alignment between 0° and 2°, versus 77.5%

(93/120) in the conventional group.The CAS group had no outliers greater than 3°.The results from these authors are comparable to our own.

The position of the components on the lateral radi- ographs displays more variability. Several factors con- tribute to this. The measurements were taken on shorter films and are hence less accurate. During surgery, an

⊡ Fig. 39-5. The virtual path of the intramedullary rod (I.M., bold black line) versus the femoral me- chanical axis (FMA, thin black line) in the sagittal plane

into account. The physiological variability is partially re- flected in these figures.On the femur,we initially followed the mechanical axis in the sagittal plane that was calcu- lated by the computer. These knees had a femoral com- ponent that was positioned more in extension than would have been the case with intramedullary alignment. Once we realized that the virtual path of our traditional refer- ence, the intramedullary rod, was in a more flexed posi- tion than the mechanical axis in the sagittal plane, we changed our desired femoral component position with CAS to 6° of flexion (

)

In the CAS group no intramedullary rod was used.

This is advantageous in cases where hardware remains

(

).As the intramedullary canal is not violated in the CAS group, one might expect less blood loss for these patients; this was not demonstrated. The explanation could be that the femoral hole that was made for the in- tramedullary rod in the conventional group was blocked with cement at the end of the procedure.

Conclusion

Correct alignment in the sagittal and coronal planes re- mains a challenge for the orthopedic surgeon performing TKA. The proven relation between malalignment and early failure is the main impetus for developing systems that avoid the outliers in alignment. From the start, it was hoped that computer-assisted surgery would fulfill this task. So far, the literature supports the use of some sys- tems, as they are clearly superior to others. Our study showed excellent results with the use of an image-based system. The advantage of image-based technology is the potential for permanent control of every step in the pro- cedure and the possibility to define landmarks kinemat- ically as well as visually on the fluoroscopic image.

The downside of these systems is the radiation hazard and the practicality of using this machinery inside an op- erating theater.New imageless systems are promising,but their clinical accuracy remains to be determined in con- trolled trials. Also, the advent of minimally invasive surgery limits the possibility of acquiring the necessary reference points for imageless CAS. Every surgeon who starts to perfom computer-assisted surgery should make his own clinical accuracy checks on the hard- and soft- ware that he uses. The tools to assess outcome are full-leg standing X-rays or CAT scanning.

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⊡ Fig. 39-6. Pre- and postoperative alignment of a patient with a large femoral prosthesis proximally, treated with a surgically navigated TKA.

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