Principles of Implantation: Measured Resection

Hiba K. Anis, MD

Trevor G. Murray, MD

Robert M. Molloy, MD

INTRODUCTION

In addition to pain relief, the goal of total knee arthroplasty (TKA) is to achieve joint stability and a balanced knee with improved function and range of motion. Malpositioned implants are currently the leading cause of failure and revision after TKA.¹ Implant survivorship was historically limited by polyethylene wear and osteolysis; however, for the contemporary TKA, instability and malalignment have been found to be the most common indications for revision.¹,² The risk of instability is unsurprising given that many stabilizing structures are removed or damaged in the process of implant positioning.²

The success of a TKA is dependent on achieving balanced and symmetric flexion and extension gaps. Accurate bone resection and precise soft-tissue balancing determine proper femoral component alignment and rotation. Although patient satisfaction is seldom correlated with normal knee kinematics after TKA, there is an abundance of literature associating instability and malrotation to poor patient outcomes and early implant failures.³,⁴,⁵ Specific clinical consequences of improper femoral rotation and alignment include anterior knee pain, patellofemoral instability, flexion gap instability, and arthrofibrosis, which in many cases necessitate revision. Measured resection and gap balancing are two different surgical techniques utilized by surgeons to achieve proper alignment and stability in TKA.

Currently, controversy exists regarding the optimal method of implantation.⁵,⁶,⁷ The measured resection technique relies on anatomic landmarks to determine femoral component positioning. With this method, fixed bone resections are made prior to soft-tissue balancing. Conversely, the gap balancing technique addresses soft-tissue balancing first whereby ligamentous releases are typically performed before the femoral cuts. Proponents of both techniques argue one is more effective than the other at achieving alignment and correct femoral component rotation. In this chapter, we will discuss the measured resection technique along with its advantages, disadvantages, and review the current evidence on this approach.

SURGICAL TECHNIQUE

The measured resection technique was developed and introduced in the 1980s by Hungerford, Kenna, and Krackow.⁸,⁹ In order to balance the knee throughout the entire range of motion, this technique emphasizes preservation of the preoperative joint line with accurate prosthetic alignment.⁹ Since the joint line position is largely preserved, adequate posterior cruciate ligament (PCL) tension is maintained, and therefore cruciate retaining implants were initially associated with the measured resection technique. However, the principles of measured resection may be used with posterior stabilized implants as well and surgeons now use either implant design with this technique.¹⁰

With this approach, fixed bone resections are made based on patient anatomy and implant dimensions. Typically, surgeons begin with the distal femoral cut followed by femoral component sizing. Then, anterior and posterior femoral cuts along with the chamfer cuts are made followed by the proximal tibial cut. Finally, after trial component implantation, ligament balance is assessed and any imbalance or laxity corrected with appropriate soft-tissue releases. In order to achieve optimal femoral component positioning and rotation, three anatomic axes derived from bony landmarks are used to guide the bone resections.

Transepicondylar Axis

The transepicondylar axis (TEA) connects the lateral epicondylar prominence to the medial epicondylar ridge (clinical TEA) or the medial epicondylar sulcus (surgical TEA). These bony landmarks mark the origin of the collateral ligaments on the femur and are therefore useful in determining the neutral rotational orientation of the femoral component. The surgical TEA is perpendicular to the tibial mechanical axis at 90° of flexion and closely approximates the optimal flexion-extension axis of the knee.¹¹,¹²

In measured resection, the femoral component is placed parallel to the surgical TEA, and several authors have found that this positioning results in ideal knee kinematics. Specifically, it improves patellofemoral tracking
as demonstrated by Miller et al¹³ who found that using the TEA as a rotational landmark resulted in the most normal patellar tracking and minimal patellofemoral shear forces. Additionally, Insall et al¹⁴ found that in patients with femoral components placed parallel to the TEA, there was a reduced incidence of femoral condylar lift off and therefore a lower risk of eccentric polyethylene wear. Furthermore, using the TEA as a reference for femoral component positioning has also been shown to consistently achieve balanced and rectangular flexion gaps.¹⁵

Anteroposterior Axis

The anteroposterior (AP) axis, also known as the Whiteside line, connects the center of the trochlear sulcus anteriorly to the center of the intercondylar notch posteriorly. The AP axis is easily identified with the knee in flexion and is perpendicular to the surgical TEA in normal patient anatomy. With the measured resection technique, the goal is to establish femoral rotation perpendicular to the AP axis by placing the femoral component perpendicular to the AP axis. The AP axis has been found to be a reliable reference for establishing correct femoral rotational alignment and improved patellar tracking, particularly in the setting of a valgus knee or when the surgical TEA is difficult to identify.¹⁶,¹⁷

Posterior Condylar Axis

The posterior condylar axis (PCA) is the third landmark commonly used with measured resection and refers to a line connecting the posterior aspects of the lateral and medial femoral condyles. In relation to the PCA, the surgical TEA is approximately 3° to 4° externally rotated in a patient with normal anatomy. Therefore, externally rotating the femoral component 3° to 4° from the PCA allows for a symmetric, rectangular flexion gap.¹⁸ This reference for femoral alignment is best used in patients with neutral or varus knees with little to no deformity.⁷ It is important to note that the PCA should be used in conjunction with other anatomic references as studies have shown that due to the variability in patient anatomy, external rotation between the surgical TEA and the PCA can range between -1° and 10°.¹⁶,¹⁹

In summary, the measured resection technique entails positioning the femoral component parallel to the surgical TEA, perpendicular to the AP axis, and 3° to 4° externally rotated from the PCA in order to achieve optimal prosthetic alignment and rotation. Unlike in the gap balancing technique, proximal tibial resections are performed independent of the femoral resections. After these fixed bone resections are complete, joint stability and balance are assessed with trial implants throughout a complete range of motion and corrected accordingly with selective ligamentous releases.²⁰ The measured resection technique often creates a flexion gap that is larger laterally; however, several authors have noted that some lateral laxity is inherent to the native knee joint and therefore many surgeons permit this residual lateral laxity.⁹,²¹,²² Furthermore, Hungerford suggests that the lateral compartment is dynamically stabilized by several surrounding structures (popliteus, iliotibial tract, biceps femoris, and lateral head of the gastrocnemius) and in fact the slight increase in flexion gap allows for rotational freedom on the lateral side which mimics normal knee kinematics.⁹

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