Three-dimensional alignment in TKR may produce better clinical results

Coronal alignment results may look good on X-ray, but patient may not always be pleased.

When it comes to alignment in total knee replacement (TKR), orthopedic surgeons may need to extend their focus beyond coronal alignment, according to one orthopedic surgeon.

William J. Hozack

“We can do a good job with coronal alignment and we [can] make our X-rays look good; however, we are not doing as good a job with the clinical results for the patients,” said William J. Hozack, MD, professor of orthopedic surgery, Rothman Institute, Thomas Jefferson University Hospital, Philadelphia. “We need to move forward to the next step, which might be the approach of using 3-dimensional alignment to get a better result.”

Traditionally, coronal alignment has been considered the critical alignment plan for TKR. The goals of coronal alignment are to: (1) restore the mechanical axis of the leg to neutral so that the center of the knee and the center of the ankle are aligned, and (2) ensure that the mechanical axis of the femur and tibia are parallel.

Coronal alignment methods

There are several ways to achieve coronal alignment, Hozack said. The traditional anatomic technique calls for a 3° varus cut on the tibia. Another technique calls for a 90° cut perpendicular to the mechanical axis of the tibia. “From a practical standpoint, it is much easier to do this,” Hozack said.

Proper coronal alignment is important to an optimal outcome. One study¹ suggested that pressure distribution, or the total load in the medial and lateral tibial compartments, changed significantly with as little as 3° of alignment variation, Hozack said. This suggests that “there was a need for precise reconstruction of the mechanical axis of the knee and that proper alignment of the tibial component was necessary.”

Survivorship studies by Merrill Ritter^2,3 and colleagues also support the theory of proper coronal alignment, Hozack said. Their results demonstrated substantially lower failure rates in the group that had overall limb alignment that was aligned to approximately 0°. Other research from Ritter’s group showed that if the tibial component was in greater than 3° of varus, the failure rate rose and survivorship dropped.³

Other important alignment factors

Although coronal alignment must be considered in every TKR, it is not the only factor. “Other than just straightforward coronal alignment, there are other things about knee replacement that ultimately determine how well it functions for the patient,” Hozack said. For example, surgeons must also take into account sagittal and rotational alignment, he said. In addition, various important anatomical landmarks, such as the epicondylar axis and the posterior condylar axis, must be considered.

“We need to move forward to the next step, which might be the approach of using 3-dimensional alignment to get a better result.”

— William J. Hozack, MD

It is clear that there is more to alignment than just a straight line in the frontal plane, Hozack said. While the surgeon may be pleased with the radiographic appearance of the knee replacement, the patient may not be satisfied. In 2005, a study published in the Journal of Bone and Joint Surgery⁴ found a wide variation in overall coronal alignment in normal, asymptomatic patients.

A recent Mayo Clinic study⁵ included about 400 patients with 14-year follow-up. The researchers examined the ± 3° window around the 0° axis. They found that the survivorship of all knees was the same.

“I think that we have come to realize that while total knee replacement works great and lasts a long time, [but] it is not perfect for our patients,” Hozack said. “We need to start thinking more in 3 dimensions about how we put things together, in terms of how we have aligned things for the knee.”

New option: 3-dimensional alignment

According to one theory of 3-dimensional knee alignment, there is a femoral flexion-extension axis, a patellofemoral axis and a tibial rotational axis.⁴ Another theory of 3-dimensional alignment suggests that the posterior-femoral condyles are cylindrically shaped, which determines the flexion-extension axis of the femur or femoral component.⁴

The best way to achieve this alignment is a hotly debated topic.

“Three-dimensional limb alignment is something that we are all struggling with in trying to get our hands around,” Hozack said. “There is a variety of different ways that this can be achieved.”

For example, some physicians use CT or MRI as an aid in positioning the femoral or tibial components, he said.

Room for improvement

Although it remains unclear if 3-dimensional alignment will improve upon existing methods, it is obvious that there is room for improvement with limb alignment. “We have an understanding that what we do now is not good enough,” Hozack said. “Patients continue to have some difficulty with the results of their knees even though everything appears to be aligned in these single dimension planes.”

Hozack hopes that as surgeons focus on 3-dimensional alignment, this will translate into a knee replacement that “feels better to the patient, not just looks better on the X-ray.”

More research will solve the limb alignment controversy, particularly the prospective trials that compare the various 3-dimensional alignment theories, Hozack said.

References

1. Werner FW, et al. The effect of valgus/varus malalignment on load distribution in total knee replacements. J Biomech. 2005; 38(2):349-355.
2. Fang DM, Ritter MA, Davis KE. Coronal alignment in total knee arthroplasty: just how important is it? J Arthroplasty. 2009;24(6 Suppl):39-43.
3. Fang D, Ritter MA. Malalignment: forewarned is forearmed. Orthopedics. 2009;32(9).
4. Eckhoff DG, et al. Three-dimensional mechanics, kinematics, and morphology of the knee viewed in virtual reality. J Bone Joint Surg Am. 2005; 87(suppl 2):71-80.
5. Parratte S, et al. Effect of postoperative mechanical axis alignment on the fifteen-year survival of modern cemented total knee replacements. J Bone Joint Surg Am. 2010; 92(12):2143-2149.

William J. Hozack, MD, can be reached at 925 Chestnut Street, Philadelphia, PA 19107; (800) 321-9999. Hozack is a consultant for Stryker and also receives royalties from Stryker.