J Korean Orthop Assoc.  2010 Oct;45(5):347-355.

An Extramedullary Femoral Alignment System in Total Knee Arthroplasty Using the Inter-Femoral Head Center Distance

• Seo JG
• Lim JS
• Lee HI
• Woo KJ

Abstract

PURPOSE
Total knee arthroplasty using the extramedullary technique for alignment has some difficulty for detecting the center of the femoral head intra-operatively. In this study we tried to evaluate the usefulness and accuracy of a newly developed Mechanical Axis Marker that synchronizes the center of the knee joint and femoral head with the mechanical axis for the distal femoral cutting and femoral prosthesis alignment. MATERIALS AND
METHODS
Between October 2008 and January 2009, 255 knees in 156 patients underwent total knee arthroplasty. We measured the distance between each centers of the femoral head using the PACS system and we applied the distance to the newly developed Mechanical Axis Marker. Subsequently, we applied the new marker to patients to align the centers of knee, the femoral head and the marker in line with the mechanical axis intra-operatively. The accuracy of the marker was validated with C-arm fluoroscopy pre-operatively in 20 patients. Post-operatively we measured and analyzed the frontal femoral component angle to evaluate the coronal alignment of the femoral implant. The accuracy was rated as excellent when the alignment was <3degrees, as good when the alignment was 3-5degrees, and as poor when the alignment was >5degrees.
RESULTS
The pre-operative validation study with the C-arm fluoroscopy showed that the distance between the femoral head center and the metal peg of the marker was within 5 mm in 95% of the patients, which implied acceptable accuracy. The average frontal femoral component angle against the mechanical axis was 89.0degrees+/-1.1 (range 86degrees-96.6degrees). The proportion of excellent, good, and poor alignments was 90.6% (231 cases), 8.6% (22 cases), and 0.8% (2 cases), respectively. The intraclass correlation coefficient between the two observers for the frontal femoral component angle was 0.972 which showed high concordance.
CONCLUSION
Our results indicate that the extramedullary technique assisted by our new Mechanical Axis Marker can easily identify the center of femoral head and improve the accuracy of frontal femoral component alignment with the proper mechanical axis.

Keyword

distance between the centers of the femoral head; mechanical axis; extramedullary alignment technique; total knee arthroplasty

MeSH Terms

Arthroplasty
Axis, Cervical Vertebra
Fluoroscopy
Head
Humans
Knee
Knee Joint
Prostheses and Implants

Figure

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  Figure 1 Mechanical axis is extramedullarily aligned using Mechanical Axis Marker. (A) X-ray shows the inter-femoral head center distance. (B) The Mechanical Axis Marker: the plastic rod and metal pegs were used to simulate inter-femoral head center distance. Black solid arrows indicate metal pegs. (C) The Mechanical Axis Marker was equipped to patients. (D) The lines indicating inguinal ligaments and the flow of femoral artery are drawn pre-operatively. (E) The mechanical axis which is the line between Metal peg (estimated femoral head center) and knee center is easily recognized during operation using this device. IHD, inter-femoral head distance.

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  Figure 2 Extramedullary alignment method using Mechanical Axis Marker, sagittal marker and two guide rods during operation.

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  Figure 3 A radiograph showing the mechanical axis in the coronal plane. The alignment of the coronal femoral components was measured by the intersection of a line drawn across the base of each component and the mechanical axis. FFA, frontal femoral angle.

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  Figure 4 Accuracy of Mechanical Axis Marker was validated using C-arm. Representative pictures are shown among 20 patients.

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  Figure 5 Acceptable range of error in Mechanical Axis Marker placement.

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