Navigation System Measures AP and Rotational Knee Laxity in ACL Replacement

Abstract

We used a non-image-based navigation system to measure anterior and rotational laxity during anterior cruciate ligament replacement. The preoperative and postoperative navigated measurements of anterior laxity were compared with the preoperative and postoperative stress radiographs. There was a significant difference between these 2 measurements, but they were significantly correlated. Navigated anterior laxity measurement can therefore be considered reliable. The intraoperative information about the correction of the anterior laxity may have relevance in controlling the quality of the procedure and improving reproducibility. Information about rotational laxity may be helpful, but its exact significance must be more precisely defined.

Anteroposterior (AP) and rotational laxity are responsible for the knee functional instability after anterior cruciate ligament (ACL) rupture.¹ The amount of anterior laxity is a diagnostic criterion² and might be a prognostic factor after reconstruction.³ Appropriate correction of AP and rotational laxity are the major goals of ACL replacement.⁴ The measurement of AP laxity by instrumental or radiographic techniques are accepted techniques of evaluation.^5,6 However, the measurement of rotational instability is not commonly performed, except in experimental studies. Navigation systems might help the evaluation of this rotational instability after ACL rupture in a routine clinical situation.

Materials and Methods

We routinely use a non-image-based navigation system OrthoPilot (B. Braun Aesculap, Tuttlingen, Germany) during ACL replacement. The detailed navigation technique has been described elsewhere.⁷ Briefly, a rigid marker is fixed on the distal femur and on the proximal tibia by a bicortical screw. A dynamic registration is performed by gently moving the knee from full extension to 90° of flexion; the respective movements of the markers are tracked by an infrared camera (Polaris; Northern Digital, Ontario, Canada), and the dedicated software can calculate the kinematic center of the knee joint. A navigated stylus registers relevant anatomic landmarks on the femur and on the tibia, both intra-articular and extra-articular: anterior tibial tuberosity, anterior tibial crest, most medial and lateral parts of the tibial plateau, tibial attachment of the posterior cruciate ligament (PCL), anterior horn of the lateral meniscus, anteromedial tibial spine, anterior part of the intercondylar femur notch, medial wall of the lateral femur condyle, most posterior point of the roof of the intercondylar femur notch, “over-the-top” position. After the kinematic and anatomic registration is completed, the software allows measurement of the respective 3-dimensional positions of the femur and tibia, and measurement of the 3-dimensional tibia displacement under the femur according the load applied by the surgeon.

We analyzed 10 cases of ACL replacement in 6 men and 4 women with a mean age of 25±6 years. All patients had clinical instability of the knee, and the diagnosis of anterior cruciate insufficiency was made by either clinical examination or magnetic resonance imaging (MRI).

The anterior laxity of the operated knee was measured after completion of the anesthetic procedure and immediately before the procedure by KT-1000 (Medmetric, San Diego, California) arthrometer testing technique at 25° of knee flexion.⁸ We only analyzed the anterior translation under maximal anterior manual force.

Arthroscopic knee evaluation was performed first to confirm the diagnosis of ACL rupture and to perform the potential meniscal treatment on request. Intraoperative navigation was performed according to the manufacturer’s recommendation. After the registration, the knee was placed at 25°±5° of flexion according to the navigation system, and the position of the tibia according to the femur reference was registered and considered as the zero position at 25° of flexion. A maximal manual anterior force was applied while keeping the knee flexion unchanged, but with no attempt to control the rotational movement. The new position of the tibia according to the femur reference was registered (Figure 1) and considered as the maximal anterior displacement of the tibia. Then, a maximal manual internal torque was applied from the zero position while keeping the knee flexion unchanged. The new position of the tibia according to the femur reference was registered (Figure 2) and considered as the maximal displacement of the tibia in internal rotation. The same measurement was made with a maximal manual external rotation torque at 25° of knee flexion.

Figure 1: Measurement of the maximal anterior tibial translation.

Figure 2: Measurement of the maximal tibial external rotation.

The same set of measurements was repeated with the knee at 90°±5° of flexion according to the navigation system.

All patients underwent an arthroscopic-assisted single-bundle ACL replacement with either a bone-patellar tendon-bone graft (2 patients) or a 4-stranded hamstring graft (8 patients). All grafts were secured with either metallic interference screw (bone-patellar tendon-bone grafts) (Fournitures Hospitalières, Heimsbrunn, France) or the TLS technique⁹ (Fournitures Hospitalières, Heimsbrunn, France) with a metallic interference screw securing a polyester braid.

The same sets of measurements that were performed before ACL replacement were performed again after having secured the graft.

The anterior laxity of the operated knee was again measured at the 4-week clinical visit by the KT-1000 technique as was done for the preoperative measurement.

The following comparisons were performed for each patient:

Preoperative KT-1000 measurement and preoperative navigated measurements of the anterior laxity at 25° of knee flexion.
Postoperative KT-1000 measurement and postoperative navigated measurements of the anterior laxity at 25° of knee flexion.
Preoperative KT-1000 measurement and postoperative KT-1000 measurement of the anterior laxity at 25° of knee flexion.
Preoperative navigated measurement and postoperative navigated measurement of the anterior laxity at 25° of knee flexion.
Preoperative navigated measurement and postoperative navigated measurement of the internal and external rotation laxity at 25° of knee flexion.
Preoperative navigated measurement and postoperative navigated measurement of the anterior laxity at 90° of knee flexion.
Preoperative navigated measurement and postoperative navigated measurement of the internal and external rotation at 90° of knee flexion.

Paired Wilcoxon test and Spearman correlation test were used at a 5% level of significance.

Results

Mean preoperative anterior laxity measured by the KT-1000 system was 13.4±4.4 mm.

Mean [SD] pre-replacement laxity measured by the navigation system was as follows:

Anterior laxity: 8.8 [2.3] mm at 25° of knee flexion and 7.7 [3.7] mm at 90° of knee flexion.
External rotation laxity: 14.2° [5.7°] at 25° of knee flexion and 17.2° [5.4°] at 90° of knee flexion.
Internal rotation laxity: 16.7° [7.1°] at 25° of knee flexion and 20.2° [8.5°] at 90° of knee flexion.

Mean [SD] post-replacement laxity measured by the navigation system was as follows:

Anterior laxity: 2.2 [0.5] mm at 25° of knee flexion and 3.8 [1.9] mm at 90° of knee flexion.
External rotation laxity: 7.3° [4.2°] at 25° of knee flexion and 13.2° [5.5°] at 90° of knee flexion.
Internal rotation laxity: 10.2° [4.0°] at 25° of knee flexion and 14° [5.8°] at 90° of knee flexion.

Mean postoperative anterior laxity measured by the KT-1000 system was 3.1±2.3 mm.

There was a significant difference between preoperative KT-1000 measurement and preoperative navigated measurement of the anterior laxity at 25° of knee flexion (P==.04). There was no correlation between these 2 measurements.

There was no significant difference between postoperative KT-1000 measurement and postoperative navigated measurements of the anterior laxity at 25° of knee flexion. There was a good correlation between these 2 measurements (r=.58).

There was a significant difference between preoperative and postoperative KT-1000 measurement of the anterior laxity at 25° of knee flexion (P=.01). There was no correlation between these 2 measurements.

There was a significant difference between preoperative and postoperative navigated measurement of the anterior laxity at 25° of knee flexion (P=.01). There was no correlation between these 2 measurements.

There was a significant difference between preoperative and postoperative navigated measurement of the rotational laxity at 25° of knee flexion (P=.01). There was no correlation between these 2 measurements. However, the direction of maximal rotation (internal or external) was not modified by one given patient.

There was a significant difference between preoperative and postoperative navigated measurement of the anterior laxity at 90° of knee flexion (P=.02). There was no correlation between these 2 measurements.

There was a significant difference between preoperative and postoperative navigated measurement of the rotational laxity at 90° of knee flexion (P=.05). There was no correlation between these 2 measurements. However, the direction of maximal rotation (internal or external) was not modified by one given patient.

Discussion

The accurate measurement of the knee laxity is a major issue during ACL reconstruction, both for planning of the reconstruction to be performed and for quality control. Preoperative and postoperative measurements are routinely performed with either dedicated instruments, such as KT-1000 or Rolimeter (Aircast, Vista, California),¹⁰ or stress radiographs.¹¹ But their accuracy may be lower than expected.^12,13 However, intraoperative measurements are not routinely performed except in experimental circumstances.¹⁴ Navigation systems have the potential to measure accurately the knee laxity during the procedure.^15,16

The navigation system used in our study has been validated as a significant help for ACL replacement¹⁷ and for performing intraoperative anterior laxity measurement.¹⁸ Our study was similar to that of Monaco et al,¹⁹ but we added a navigated laxity measurement at 90° of knee flexion and post-reconstruction measurements. We confirmed that the navigation system allowed measuring anterior and rotational laxity during ACL replacement. The preoperative and postoperative navigated measurements of the anterior laxity were significantly different from the preoperative and postoperative KT-1000 measurements. However, this difference was <2 mm in most cases and may be considered clinically irrelevant. Furthermore, the 2 sets of measurements were significantly correlated, and the agreement between these 2 measurements was considered as good. The intraoperative laxity measurement can therefore be considered as reliable.

The most relevant feature of this study was to measure the rotational laxity of the knee before and after ACL reconstruction. Rotational laxity has been measured in experimental settings,²⁰ but the in vivo measurement is demanding. Navigation systems may allow measuring the rotational laxity of the knee on a routine basis. Zaffagnini et al²¹ performed only a feasibility study, without providing detailed results. We confirmed the feasibility of this measurement with the navigation system used, and assessed the rotational laxity of this study group before and after ACL reconstruction. Results in absolute values were similar to those of previous studies.^20,21

However, this study does have drawbacks. The sample is small, and a larger sample may bring a larger variation. There was no calibration of the anterior force or the rotational torque during navigated measurement, which may induce significant bias. There was no navigated measurement on the contralateral knee, which can only define the reference laxity. Such measurement would be invasive and unethical; there is currently no validated noninvasive technique using the navigation system. The postoperative KT-1000 measurement was performed after 4 weeks, and the graft may have elongated during this period; however, according to the literature, significant elongation is not likely.

The intraoperative information about the correction of the anterior laxity may have relevance in controlling the quality of the procedure and improving reproducibility. It may help define the appropriate tension of the graft and determine whether additional intra-articular or extra-articular procedures may be useful. Furthermore, information about rotational laxity may be helpful, but its exact significance must be more precisely defined.

Conclusion

Correction of the anterior and rotational laxity is the main goal of ACL replacement. Both measurements are critical for quality control of the procedure, and both may provide intraoperative information relevant to adapting the procedure to the patient’s needs. The navigation system used in this study allows an accurate and reliable measurement of the anterior laxity during ACL replacement, which helps control the quality of the reconstruction and provides data for modifying the procedure or performing additional procedures if laxity is not fully corrected. The intraoperative assessment of the rotational knee laxity is a new possibility offered by navigation. This additional information might be particularly helpful when considering single- or double-bundle ACL reconstruction, or it may be useful to improve the quality of ACL reconstruction. However, the relevance of this information is still questionable, in that few techniques are currently used to measure rotational laxity in a clinical setting. Development of such devices is necessary to control the usefulness of this new information.

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Author

Dr Jenny is from the Hôpitaux Universitaires de Strasbourg, Centre de Chirurgie Orthopédique et de la Main, Illkirch-Graffenstaden, France.

Dr Jenny receives royalties from and is a consultant for B. Braun Aesculap.

Correspondence should be addressed to: Jean-Yves Jenny, MD, Hôpitaux Universitaires de Strasbourg, Centre de Chirurgie Orthopédique et de la Main, 10 avenue Baumann, F-67400 Illkirch-Graffenstaden, France.

doi: 10.3928/01477447-20090915-56