Principles of Instrumentation and Component Alignment

Anthony P. Gualtieri, MD

Jonathan M. Vigdorchik, MD

Ran Schwarzkopf, MD, MSc

INTRODUCTION

Current instrumentation for total knee arthroplasty (TKA) facilitates the ability of the surgeon to make reproducible and accurate bone cuts that consistently restore the mechanical axis of the limb. Additionally, instruments may have the versatility to make adjustments that can accommodate for bone deficiency/deformity, ligament imbalance, and anatomic variations. This chapter focuses on the role and application of instruments in TKA.

NORMAL ANATOMY

There is great variation in native limb alignment. Individual differences in height, weight, and bone morphology affect the static knee alignment, and these differences are further affected by eccentric and asymmetric degenerative changes that are typically associated with the arthritic knee.

The mechanical axis of a knee normally aligned in the coronal plane is defined as a line drawn from the center of the femoral head passing through the center of the knee and ending in the center of the ankle joint. The anatomic axis refers to the intersection of a line down the axis of the femoral and tibial shafts (Fig. 32-1). On average, the anatomic axis of the knee is 5° to 7° of valgus. This angle represents a combination of the valgus alignment for the femoral condyles (a mean of ˜7°) and the varus tilt of the tibial plateau (a mean of ˜3°).¹

The axis of rotation of the femur (flexion) also has a wide deviation, as the amount of the posterior condyles that falls below the transepicondylar axis can vary greatly. This is important if we make a transverse cut on the tibia and want to establish a rectangular space in flexion during TKA.

The flexion axis of rotation is complex but generally believed to transect the medial and lateral epicondyles at the origins of the collateral ligaments. It is transverse to the long axis of the tibia. At 90° of flexion, the medial condyle extends approximately 3 mm (1 to 6°) below (more posterior) the lateral condyles (Fig. 32-2).

BIOMECHANICS

In a limb with anatomic alignment of 7° of valgus, during normal gait, 60% to 70% of weight-bearing forces in stance phase pass through the medial compartment of the knee. Small changes in alignment can lead to substantial changes in load distribution in each of the compartments, which ultimately may predispose to the joint to developing osteoarthritis.²,³,⁴,⁵ This may explain why there is often asymmetric chondral degeneration noted with progressive varus or valgus deformity in osteoarthritis.

Restoration of limb and component alignment during TKA normalizes the distribution of forces across the implant and enhances implant survival and performance. Lotke and Ecker⁶ first established the overall importance of limb alignment and subsequent balance of soft tissues to optimize the results of total condylar knee replacement. Hsu et al⁷ demonstrated that a 5-degree axial malalignment can change the load distribution up to 40%. These studies were corroborated by Ritter et al,⁸ who showed that early failures occurred with tibial varus of more than 5°.

THEORIES OF AXIAL IMPLANT ALIGNMENT

The potential for errors of component implantation is great when we consider that the femoral and tibial components each have 6° of freedom in which they can be implanted: varus-valgus tilt, flexion-extension, proximal-distal position, internal-external rotation, anterior-posterior translation, and medial-lateral translation. The overall limb alignment and patella each has 3° of freedom. Combining all of the possibilities of implanting the components in relationship to one another, there exist more than 11,000 ways to get component alignment wrong. Fortunately, proper placement and alignment of components can be thought of as a range of satisfactory positions; however, implantation errors can be minimized by adhering to proven principles of TKA.

Restoring the mechanical limb alignment to neutral is a primary goal of TKA. There are three primary techniques for achieving alignment in TKA: mechanical alignment; anatomical alignment; and kinematic alignment. While mechanical and anatomical alignments were first proposed in the 1980s, kinematic alignment was only described in the mid 2000s. In order to make an informed decision on which technique to utilize, it is important to understand the philosophies that guide each surgical technique.

FIGURE 32-1 The reference axes for correct alignment are the anatomic axis along the shaft of the bone and the mechanical axis from the femoral head to the center of the ankle.

Mechanical Alignment

The mechanical alignment for TKA was first described by John Insall in 1985.⁹ The mechanical axis, as discussed previously, passes from the center of the femoral head to the center of the ankle joint. At the level of the knee, the mechanical axis typically passes just medial to the tibial spine.¹⁰ Mechanical alignment depends on the distal femoral cut and the tibial cut, both being made perpendicular to their respective mechanical axes. This translates to a distal femoral cut of ˜6° of valgus and a tibial cut of ˜0° of varus/valgus.⁹

The philosophy behind this technique was based on Insall’s belief that anatomically aligned knees were at risk for medial tibial plateau fixation failure due to the already established fact of increased forces across the medial aspect of the joint.²,³,⁴,⁵,⁹ Insall pointed out that although the loading forces were even in the medial and lateral compartments during stance, these loading forces would be uneven during gait because of a laterally directed ground reaction force.⁹,¹¹

FIGURE 32-2 In flexion, the posterior medial condyle falls approximately 3° (3 mm) further below the transepicondylar axis (TEA) than the lateral posterior condyle. The medial tibial plateau is approximately 3° (3 mm) more distal than the lateral tibial plateau. If you make a transverse tibial osteotomy, to maintain a balanced rectangular flexion space, you will resect more bone from the medial posterior condyle.

Anatomical Alignment

The anatomic axis of the lower extremity is defined by the intramedullary (IM) canals of the femur and tibia. As described above, the anatomic and mechanical axes of the tibia match, while the anatomic axis of the femur is more valgus than the mechanical axis by an 8° to 9° angle.¹,¹⁰ Additionally, it is critical to remember that the normal knee joint is aligned at 2° to 3° of varus relative to the mechanical axis of the lower limb.¹ Therefore, the overall alignment of the lower extremity is a combined 5° to 7° of valgus.¹,¹⁰

Hungerford and Krackow described the anatomical alignment technique for TKA in 1985.¹² The goal of anatomical alignment is to anatomically recreate the joint line. The tibia is cut at 2° to 3° of varus to the anatomical/mechanical axis of the tibia. The distal femoral cut is made at 8° to 9° of valgus, which is that same anatomical/mechanical difference mentioned previously. The overall alignment of the implanted components is therefore approximately 6° of valgus, which roughly matches the normal tibiofemoral angle of 5° to 7°.¹²,¹³ This anatomic alignment technique purportedly improves the load distribution in the tibial component and improves patellar biomechanics by reducing lateral retinacular ligament stretching in knee flexion.¹⁴,¹⁵

Issues associated with the anatomical alignment technique include the difficulty of obtaining a tibial cut at 2° to 3° of varus and creating an oblique joint line. Consistently cutting the tibia at 2° to 3° requires a level of precision that may only be available through computer-assisted technologies. Creating an oblique joint line, rather than one parallel to the ground, has risks of higher chance of early failure.⁸

Kinematic Alignment

Kinematic alignment technique is the newest of the three techniques for TKA, first being performed in the mid-2000s. The notion of kinematic alignment for TKA is dependent on the work of Hollister and colleagues, who performed much of the original research on kinematics of the knee.¹⁶ Additionally, much of the basic concepts of kinematic alignment hinges upon the technique of anatomical alignment, specifically the objective of reconstituting normal knee kinematics through resecting and replacing as little bone as possible.¹²,¹⁷ Kinematic alignment is advocated as being patient-specific and ligament sparing.

Critical to the understanding of the kinematic alignment technique for TKA is the understanding of the three axes of movement in the knee. These three axes describe the movement of the patella and tibia in relation to the femur:

The first and primary axis is a transverse axis that passes through the center of a circle fit to the articular surface of the femoral condyles from 10° to 160° of flexion. This is the axis about which the tibia flexes and extends.
The second is a transverse axis that describes the motion of the patella as it flexes and extends around the distal femur. This axis is parallel, proximal, and anterior to the primary transverse axis.
The third axis is longitudinal and centered in the tibia and is that about which the tibia internally and externally rotates around the distal femur. It is perpendicular to the first and second axes.

The main goal of kinematic alignment for TKA is to match the transverse axis of the femoral component to the primary transverse axis of the femur about which the tibia flexes and extends.¹⁶,¹⁸ By aligning these axes and restoring the native tibial-femoral articular surfaces, the kinematic alignment method uses bony resection to maintain the native resting lengths of the collateral, retinacular, and posterior cruciate ligaments, thereby foregoing any need for ligamentous releases.¹⁸

Although the kinematic alignment technique proposes improved clinical outcomes through restoring native knee kinematics, there are some risks associated with this technique. It is a relatively new technique, so long-term survivorship data are sparse. Additionally, as mentioned previously, the varus tibial positioning inherent to this technique is a theoretical risk factor for earlier failure.⁸

MECHANICAL ALIGNMENT

Outcomes

The mechanical alignment technique has been the mainstay of TKA for decades. Mechanical alignment aims to align the hip-knee-ankle angle of the limb in neutral, thereby achieving a more balanced load distribution in the medial and lateral compartments. This classical approach has shown predictably good results and reliably high survivorship data.¹⁹,²⁰,²¹,²²,²³ A 2009 retrospective survivorship analysis of over 6000 TKAs showed that postoperative alignment was the chief predictor of failure and revision surgery, regardless of preoperative alignment. This study produced an “ideal” coronal alignment of 2.4° to 7.2° of valgus which was associated with the best overall survivorship.²² An older study by Jefferey et al reported that when the mechanical axis fell in the medial 1/3 of the TKA, the rate of aseptic loosening was markedly lower than when the mechanical axis was further medial or lateral from this neutral position (3% vs. 24%).²³ Another prospective study on mechanical alignment using computer-assisted surgery found that patients with a postoperative alignment within 3° of a neutral axis had increased International Knee Society Score and Short-Form 12 Physical Scores at both 6 weeks and 12 months postoperatively.²⁴ In summary, these extensive data contribute to our knowledge that when performed well, the mechanical axis technique for TKA yields excellent results and survivorship.

Unfortunately, the method of mechanical axis alignment is associated with a persistent population of dissatisfied patients. A study of 1703 patients in Canada found that approximately 20% of patients were dissatisfied with their TKA.²⁵ These data were echoed by a US report of more than 10,000 patients, which showed 18.2% dissatisfaction.²⁶ Across numerous studies, these results have been reinforced, with 11% to 18% of patients remaining dissatisfied with their TKA.²⁷,²⁸,²⁹,³⁰,³¹,³²,³³,³⁴ The etiology for this dissatisfaction has been multifactorial, but possible causes could be alteration in limb alignment from natural valgus to a postoperative neutral position and collateral ligament tensioning changes.³⁵ Ultimately, it seems that while a large proportion of patients are satisfied after TKAs performed by the surgical technique of mechanical axis alignment, somewhere between one in every five to six patients are dissatisfied with their results.

Instrumentation

Classically, extramedullary and IM instruments have been used for the tibial and femoral osteotomies in the mechanical alignment technique. Both extramedullary and IM guides have been shown to be equally effective for the tibial osteotomy (Fig. 32-3).³⁶ For the femoral osteotomies, both extramedullary and IM guides have been utilized, although IM guides have been preferred due to studies
showing improved relative accuracy.³,³⁷,³⁸,³⁹,⁴⁰,⁴¹,⁴²,⁴³ Additionally, there are two important femoral osteotomies. The distal osteotomy, which relies on extramedullary or IM guides, sets axial alignment and the anterior-posterior osteotomies that determine rotational alignment, which will be discussed in a later section.

FIGURE 32-3 Intramedullary (A) or extramedullary (B) guides may be used to make a transverse osteotomy of the tibia.

For the proximal tibial osteotomy, the extramedullary guide is placed parallel to the tibial crest in the coronal plane, with ability to adjust for a posterior slope. Extramedullary systems bypass any potential deformity of the tibial shaft, which can bias alignment. They also reduce the risk of fat embolism syndrome and allow for easy adjustment of alignment in all planes. Although this technique can be highly effective and accurate, Cates et al³⁷ reviewed alignment of the proximal tibial cut with the use of extramedullary guides and found a significant percentage of alignment errors. In morbidly obese patients, in whom external landmarks are obscure, these guides have a greater potential for error.

IM guides can also create reproducible osteotomies.⁴⁴,⁴⁵ This technique is applicable for most knees, except when there is significant tibial deformity or obstruction of the tibial canal by previous fracture or hardware. IM instrumentation is placed through a drill hole in the tibial plateau. The pilot hole is often started at the junction of the insertion of the anterior cruciate ligament and the anterior horn of the lateral meniscus. Proximally, the drill hole should be wide enough so the guide is not biased at this level and marrow pressures can be released during rod insertion. It has been shown that a 12.7-mm drill hole can reduce the risk of intraoperative oxygen desaturation when using an 8-mm alignment rod designed to allow venting of the canal.⁴⁶ It has also been shown that fluted and hollow rods significantly reduce IM pressures within the canal,⁴⁷ offering an explanation in the mechanism of reduced fat embolization.

Both IM and extramedullary guides have telescoping elements that are used to place a cutting block at the desired resection level (Fig. 32-4). The varus and valgus alignment is adjusted to parallel the mechanical axis of the tibia, and the posterior slope is “dialed in” by the cutting block to approximate the native posterior slope. The block is pinned at an appropriate resection level and
the osteotomy is performed. A measuring guide (stylus) can be used to accurately determine the amount of bone to be resected, generally seeking to remove approximately 10 mm (depending on the implant and anatomy) of bone and cartilage from the less arthritic hemiplateau. Adjustments in placement of the stylus may be necessary in the presence of subchondral loss and abnormal contouring of the plateau.

FIGURE 32-4 A tibial cutting block is fixed in place and used to guide the transverse tibial osteotomy.

For the distal femoral osteotomy, mechanical alignment technique dictates the osteotomy be aligned in 4° to 7° of valgus. IM and extramedullary guides are available for the distal femoral resection. However, most surgeons find that the IM devices are reproducible, easy, and applicable in the vast majority of cases (Fig. 32-5).³,³⁷,³⁸,³⁹,⁴⁰,⁴¹,⁴²,⁴³ Cates et al³⁷ reviewed 200 consecutive TKAs in which IM femoral guides were used in 125 cases and extramedullary femoral guides in 75 cases. The distal femoral resection angle was outside the accepted range (4° to 10° of femoral valgus) in 28% of the extramedullary group and in only 14% of the IM group. Joint line orientation was also outside the normal range twice as frequently in the extramedullary alignment group. The authors suggested that IM guides improve the accuracy of distal femoral osteotomy. Teter et al³⁸ reviewed 201 TKAs in which IM
femoral guides were used. Distal femoral alignment was considered inaccurate in only 8% of the x-rays. Risk factors for inaccurate alignment included capacious femoral canals and distal femoral bowing. Regardless of the technique, IM or extramedullary, attention to detail and confirmatory assessments are critical in minimizing alignment errors.

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