Knee Surgery, Sports Traumatology, Arthroscopy

CT MEASUREMENTS PRIOR TO COMPUTER-ASSISTED TOTAL KNEEARTHROPLASTY DO NOT IMPROVE ROTATIONAL PLACEMENT OF THEFEMORAL COMPONENT A de Ladoucette Clinique de l'Union Boulevard

RATALENS 31240 SAINT JEAN Email : aymard.deladoucette@clinique-union.frCorresponding author: A de Ladoucette

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Manuscript Number: KSST-D-11-00377R3

Full Title: CT MEASUREMENTS PRIOR TO COMPUTER-ASSISTED TOTAL KNEEARTHROPLASTY DO NOT IMPROVE ROTATIONAL PLACEMENT OF THEFEMORAL COMPONENT A de Ladoucette Clinique de l'Union BoulevardRATALENS 31240 SAINT JEAN Email : aymard.deladoucette@clinique-union.frCorresponding author: A de Ladoucette

Article Type: Original Paper

Corresponding Author: aymard de ladoucette, MDclinique de l'UnionSaint Jean, FRANCE

Corresponding Author SecondaryInformation:

Corresponding Author's Institution: clinique de l'Union

Corresponding Author's SecondaryInstitution:

First Author: aymard de ladoucette, MD

First Author Secondary Information:

Order of Authors: aymard de ladoucette, MD

Order of Authors Secondary Information:

Abstract: Purpose: The aim of this study is to determine whether computer-assisted surgery(CAS) can gain more precision by utilizing information from pre-operative computedtomography (CT).Methods: Sixty-five patients undergoing total knee arthroplasty in 2008 were included.On pre- and post-operative CT, epicondylar and posterior condylar lines were drawnand the posterior condylar angle (PCA) between these two lines measured. During thesurgery, epicondylar and posterior condylar lines were also registered before and afterCAS-guided cuts were made. CAS was used to fix the orientation of the cutting guideon the distal femur at 3° of external rotation from the posterior condylar line.Results: There was no correlation between CT measurements and CASmeasurements. All orientations of the lines (relative to the horizontal) and anglesmeasured using CT were significantly different (p<0.05).Conclusion: No relationship was found between pre-operative CT measurements andperi-operative CAS measurements used to orient the femoral component. Dataobtained from CT cannot be used peri-operatively with CAS to improve rotationalsetting of the femoral component.Level of evidence: Therapeutic study. Level II.

Key words: computed tomography, computer assisted surgery, femoral rotation,epicondylar line, posterior condylar angle

Response to Reviewers: I have followed the advices from the reviewer and the manuscript had been correctedby SF edit. I have modified the tables and references according to both their advices. Ihope the manuscript now fullfill all the requirements needed for an acceptance.Yours sincerely

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CT MEASUREMENTS PRIOR TO COMPUTER-ASSISTED TOTAL KNEE

ARTHROPLASTY DO NOT IMPROVE ROTATIONAL PLACEMENT OF THE

FEMORAL COMPONENT

A de Ladoucette

Clinique de l’Union

Boulevard RATALENS

31240 SAINT JEAN

Email : aymard.deladoucette@clinique-union.fr

Corresponding author: A de Ladoucette

Title Page

CT MEASUREMENTS PRIOR TO COMPUTER-ASSISTED TOTAL KNEE

ARTHROPLASTY DO NOT IMPROVE ROTATIONAL PLACEMENT OF THE

FEMORAL COMPONENT

Introduction

Outcome following total knee arthroplasty (TKA) depends on rigorous placement technique

[4, 29]. Rotational misalignment of the femoral component can cause anterior patellar or

medial femoro-tibial pain [11, 12, 15]. The femoral component must be implanted in external

rotation [10, 11, 19, 25]. The most acknowledged landmark for evaluating femoral rotation

remains the epicondylar line (ECL) [6, 16, 19, 27, 30].

Computed tomography (CT) allows reproducible and reliable measurement of femoral

epiphysis rotation before and after surgery [6, 8, 26]. Computer assisted surgery (CAS) is

reliable in the frontal alignment of the knee [7, 14, 31]. A few studies analyzed the

contribution of CAS to the rotational alignment of the femoral component [8, 19, 22, 23, 26].

CAS helps to visualize the same lines and measure the same angles as measured by CT. CAS

and CT give measurements that are accurate within one degree.

The aim of the present study is to determine whether there is a correlation between CT and

CAS measurements. If CAS is accurate as a guide for the rotation of the femoral component,

post-operative CT measurements of distal femur rotation should be predicted from pre-

operative CT and from pre- and post-cut (peri-operative) CAS measurements.

Materials and methods

All patients (n=65) undergoing TKA in 2008 with CAS using the Navitrack system

(Orthosoft, Montreal, Canada) at our institution were prospectively included.

CAS with the Navitrack system

The Navitrack (Orthosoft, Montreal, Canada) navigation system gives an image-free

application dedicated to the Natural Knee II prosthesis (NKII) (Zimmer, Warsaw, IN) [7,

31]. The version of Navitrack used in this study was exclusively developed for the NKII. The

use of the Navitrack system has been described previously [7, 21, 31]. Bolognesi et al. [7]

reported that, using this navigation system, 98% of the femoral components and 100% of the

*Blinded ManuscriptClick here to view linked References

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tibial components were placed within 3 degrees of the ideal radiographic goal position (90 ±

3°).

Surgical technique

The Natural Knee II approach, with the conventional ancillary system, was used to externally

rotate the femoral component at 3° from the posterior condylar line. This follows the

principles of the anatomical surgical technique [18]. While operating using CAS, after having

registered the posterior condylar line, the same principle were followed and, using CAS, the

cutting guide was systematically adjusted to 3° of external rotation from the posterior

condylar line. When CAS indicated that the cutting guide lay at the appropriate orientation,

the cutting guide was securely fixed to the bone and the cut was made. Before performing the

cut, it was always verified that this orientation was not markedly different from the orientation

that would have been applied if the surgery were performed without CAS using conventional

ancillary tools.

Radiographic evaluation

CT was performed pre-operatively and at three months post-procedure in order to measure the

rotation of the distal femur according to the protocol described by Hernandez et al. [13] and

Mahboubi et al. [20]. The posterior condylar line (CT PL) and the epicondylar line (CT ECL)

were measured according to the methods of Michaut et al. [21] (Fig. 1). The posterior

condylar angle (CT PCA), between the posterior condylar line and the epicondylar line, was

calculated as described by Richard et al. [24] (Fig. 1). Separate measurements were made by

an experienced radiologist and by the author. When a difference in measurement between the

two reviewers was noted, a common verification was undertaken.

CAS evaluation

Landmarks used for the registration of lines and angles with CAS were the same as those used

with CT (Fig. 2). The orientations of the posterior condylar line (CAS PL) were measured, as

it is known that registration of the epicondylar line is not accurate with CAS [7, 19]. The

rotation of the cutting guide from the posterior condylar line (CAS ER) was also registered,

just after fixing the cutting guide onto the bone. For each of these measurements a positive

result indicated an externally rotated angle.

Statistical analysis

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Pre- and post-operative quantitative variables on CT and quantitative variables measured peri-

operatively by CAS were compared using both Student’s parametric test and comparison of

variances (Levene’s and Bartlett's tests). Analysis of linear correlations between the results of

the CT and CAS measurements was performed using the Pearson test. Data were analyzed

using XLSTAT 2009 software (Addinsoft). Significance was accepted at p<0.05.

Results

Table 1 shows results of CAS and CT posterior condylar line orientation. The pre-operative

measurements of the posterior condylar line from CT differed significantly from those

obtained using CAS (p<0.001). Comparison of post-operative CT measurements with the

same post-operative CAS measurements indicates a significant difference (p<0.001).

Table 2 reports results of pre- and post-operative measurements. The post-CAS PL, measured

after cutting, is no more externally orientated than the rotation of the femoral epiphysis

measured with CAS before cutting (pre-CAS PL) (n.s.). The pre-operative rotation set to the

cutting guide with CAS (CAS ER) is statistically similar to the orientation of post-CAS PL

(n.s.). The post-operative CT posterior condylar angle (post-CT PCA) values were lower than

the pre-operative values (pre-CT PCA) but the difference was not statistically significant

(n.s.).

No significant correlations were found between the pre-operative and post-operative CT

measurements of the posterior condylar angles (Fig. 3).

Discussion

The most important finding of this study is that the femoral component is externally rotated

with CAS in patients treated with TKA. CAS was used according to the anatomic technique,

as would have been done with the conventional NKII ancillary [18]. A systematic external

rotation at 3° to the condylar posterior line was given to the antero-posterior femoral cutting

guide. It has long been known that the posterior condylar cut must not be parallel to the

posterior condylar line but must be more or less parallel to the epicondylar line, to favor

ligament balancing and patellar tracking [1, 3, 11, 19, 21, 25].

Location of the epicondylar line by CAS is neither accurate nor reliable [7, 19]. However,

Arima et al. [3] and Stöckl et al. [26] easily and accurately marked the posterior condylar line

using CAS. Moreover, CT measurement of the epicondylar and posterior condylar lines are

precise both pre- and post-operatively [6, 8, 21, 26, 27]. The PCA described by Richard et al.

[24] is the angle determined by CT between the epicondylar line and the posterior condylar

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line. This angle indicates precisely the distal femoral rotation [12, 21, 24, 29]. The anatomic

epicondylar line is the most reliable landmark for determining the orientation of the femoral

component [2, 4, 5, 9, 12, 17]. However, CAS inaccurately registers the epicondylar line [7,

15, 19]. As it was necessary in this study to link pre-operative CT measurements with peri-

operative CAS measurements; the posterior condylar line was the only line that could be used

both with CAS and with CT [3, 26]. As the posterior condylar angle is between the posterior

condylar line and the epicondylar line, only the posterior condylar angle could be used to link

pre-operative CT measurements with peri-operative CAS measurements.

This study was conducted to determine whether rotation of the femoral component set by

CAS before cutting was concordant with rotation measured on the post-cutting CAS and on

post-operative CT. Results confirmed that the measured post-cutting CAS external rotation is

statistically comparable to the pre-cutting CAS rotation, implying that what is set with CAS

before cutting matches the outcome after cutting.

The second aim was to determine whether the difference of CT posterior condylar angles

measured pre- and post-operatively was equivalent to the rotation applied peri-operatively

with CAS (CAS ER ).

The measured post-operative CT external rotation was smaller than the rotation that should

have been obtained if the CAS and CT measurements correlated. These results indicate a lack

of correlation between lines and angles measured by CT versus CAS even though the same

pre-operative landmarks were used with both. This absence of correlation is partly due to the

fact that CT can only measure bone references whereas CAS takes cartilage into account,

although the most prominent points of the posterior condyles are registered by both CT and

CAS. In osteoarthritis, posterior condylar cartilage wear is not as severe as distal condylar

cartilage wear. The normal postero-medial cartilage thickness is greater than that of the

postero-lateral compartment [28].

Another explanation for the difference between CT and CAS measurements is the fact that the

two systems use different reference planes. The CT reference is the true horizontal plane

whereas CAS uses a computer-calibrated plane at the onset of surgery. This calibration is

evaluated at the first stage of registration of the calibration CAS tools, from the plane set

while determining the center of the femoral head, and from the flexion/extension plane of the

knee.

The PCA measured with CAS is of no use. There is a lack of precision in recording

epicondylar reference points because of the presence of ligament and synovial tissues that

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cover the bone structures [7, 19]. The everted or tilted patella hides the lateral epicondylar

landmark and total exposure of the lateral condyle remains difficult [3, 6, 8, 21, 26].

The anatomical surgical technique to determine orientation of the femoral component was not

improved with the CAS system used in this study, contrary to the conclusion of Michaut et al.

[21] who used the same Navitrack CAS system and correlated it with CT measurements.

However, this is the first study to precisely measure and compare the posterior epicondylar

and condylar lines with CAS before and after condylar cuts with the pre- and post-operative

CT measurements. Several authors reported that CAS successfully directs the femoral

component in external rotation [8, 21, 22, 23, 26] as confirmed here.

This study has some limitations. First, it was designed to compare two systems that reference

several anatomical landmarks essential to the performance of TKA. These two systems, CAS

and CT, do not use the same reference plane, even if the lines and angles used refer to the

same landmarks. The data used for this study are exactly those provided before surgery by CT

or to the surgeon intraoperatively by CAS. This study did not compare the two techniques but

determined whether these two methods of measuring the torsion of the femoral distal

epiphysis could be used together during surgery. As noted by Lützner et al. [19], who used

another CAS system, the Navitrack CAS system used in this study does not offer more

precision than the usual surgical system without CAS in handling rotation of the femoral

component. What is measured peri-operatively with CAS is different than the pre- and

postoperative CT measurements. Yet, CAS has proven its efficacy for the alignment of the

inferior limb on the frontal and sagittal planes. It seems unnecessary to utilize pre-operative

CT to evaluate distal condylar torsion as it will have no use during the procedure, even with

CAS to help align the posterior cutting guide.

Conclusion

Pre- and post-operative CT was performed on 65 patients operated on using CAS for TKA.

The femoral component was to be set with an external rotation of 3° from the pre-operative

posterior condylar line. The cuts were controlled by CAS. The aim of this study was to

explore the relationship between pre-operative CT and peri-operative CAS measurements. No

correlations could be found between the lines and angles measured by CT and those measured

by CAS. CAS helps to externally rotate the femoral component but the external rotation

measured by CAS is totally different from that measured by CT. Pre-operative CT is of no

value in determining the exact rotation of the femoral component needed during CAS-guided

surgery to ensure that post-operative rotation complies with the preoperative goal.

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Figure 1Click here to download high resolution image

Figure 2Click here to download high resolution image

Figure 3Click here to download high resolution image

Pre- PL Post- PL

CT (°) 2 ± 9 1 ± 7

CAS (°) 2 ± 3 3 ± 2

p <0.001 <0.001

Table 1: Median ± standard deviation pre- and post-operative posterior condylar line (PL)

measured on computer tomography (CT) and with computer assisted surgery (CAS). For the

CAS measurements, ‘pre-’ indicates before the cut and ‘post-’ after the cut. Comparison of

variance (Levene test) (p<0.05). *The orientation of PL was calculated in relation to the

horizontal plane.

Table 1

CT PCA CT PL CT ECL CAS PL CAS ER

(°) (°)* (°)* (°)* (°)*

Pre-(operative) 2 ± 3 2 ± 9 3 ± 9 2 ± 3 3 ± 2

Post-(operative) 1± 4 1 ± 7 2 ± 6 3 ± 2

p n.s. n.s. n.s. n.s. n.s.

Table 2: Median ± standard deviation of posterior condylar angle (PCA), posterior condylar

line (PL), and epicondylar line (ECL) measured on computer tomography (CT) and with

computer assisted surgery (CAS). For the CAS measurements, ‘pre-’ indicates before the cut

and ‘post-’ after the cut. Pre-operative CAS rotation of the cutting guide (CAS ER) was

compared to post-CAS PL. Comparison of variance (Levene test) (p<0.05). *The orientation

of ECL, ER, and PL were calculated in relation to the horizontal plane.

Table 2