Sizing and Balancing: Gap Technique versus Measured Resection

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CHAPTER SYNOPSIS

The surgical technique in total knee arthroplasty (TKA) has continued to evolve from the initial techniques of the modern total knee replacement as described by John Insall. From the two camps of posterior cruciate sacrificing and posterior cruciate retaining TKA systems came two theories of surgical technique: gap balancing with classic alignment and measured resection with anatomic alignment, respectively. Improved understanding of the surgical pitfalls as well as implant improvements have led to a gradual integration of the two techniques to provide reliable clinical results regardless of the type of implanted knee replacement.

IMPORTANT POINTS

1

The goals of TKA are to return the prosthetic knee joint to a near neutral mechanical alignment in order to place equal stress on each tibiofemoral compartment.
2

Gap balancing has traditionally been associated with posterior cruciate substituting total knee implants. The basic principles are to produce equal flexion and extension gaps by performing soft tissue releases before consideration of bony resection.
3

Rotation of the femoral component must be accurately assessed as it may produce an asymmetric flexion gap as well as lead to patellar maltracking.
4

Sagittal balancing is performed with either anterior or posterior referencing with the pitfalls being notching/overstuffing the anterior compartment versus overresection/underresection of the posterior condyles.
5

Measured resection has been linked with posterior cruciate retaining implant designs. A “measured resection” is performed to resect equal amounts of bone and replace with implant thickness. The joint line should be restored to the prearthritic level to have the posterior cruciate ligament function normally.
6

In most systems, these two surgical theories have been integrated to maintain the joint line at its anatomic location, while balancing the flexion and extension gaps equally.

CLINICAL/SURGICAL PEARLS

1

Regardless of surgical technique, the mechanical alignment after total knee replacement should be placed in neutral overall alignment with the tibial resection performed in neutral and 5 to 7 degrees of valgus alignment for the distal femoral resection.
2

Femoral rotational alignment should be assessed accurately, and it may be necessary to use more than one technique to accurately confirm rotation in certain circumstances.
3

With the gap balancing technique, if the flexion and extension gaps are not balanced, one should assess soft tissue balancing prior to performing an additional bony resection.
4

Regardless of anterior or posterior referencing, to obtain sagittal balance, one should optimize sagittal component sizing without overresection of the posterior condyles as well as preventing notching or overstuffing the anterior compartment.
5

In measured resection technique, the bony resection depth should equal the implant thickness. The main goal is to restore the joint line to its anatomic location.
6

A posterior cruciate ligament that is too loose will be incompetent and may not provide the anatomic femoral roll back required to obtain full flexion.
7

A “tight” posterior cruciate ligament will produce increased stresses posteriorly. This should be recognized intra-operatively with the trial components in place, where there will be “lift-off” of the trial insert with flexion.
8

With the integration of techniques, regardless of implant design, a measured resection is attempted, followed by assessment of the soft tissues to balance the flexion-extension gaps.

Clinical/Surgical Pitfalls

1

One should be comfortable with several methods to assess femoral rotation, as reliance on one technique could lead to surgical errors, especially with anatomic abnormalities.
2

When sizing a total knee implant in the sagittal plane that is in between sizes: with posterior stabilized implant systems, one should avoid downsizing the implant size and overresecting the posterior condyles. In cruciate retaining designs, one should avoid upsizing to the larger size, as this would create a tight flexion gap that may require partial posterior cruciate ligament release.

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HISTORY/INTRODUCTION/SCOPE OF THE PROBLEM

The continued evolvement of total knee arthroplasty (TKA) has led to its long-term clinical success and durability. The surgical technique has been refined as well as improvements in implant fixation, design, and inventory. Current techniques to optimize patient recovery, including “less invasive surgical techniques” and perioperative pain control protocols, have improved the short-term results; however, the modern surgical technique pioneered by John Insall continues to provide the basic principles toward a successful, durable TKA.

ANATOMIC KNEE ALIGNMENT

The evaluation of knee alignment can be assessed in a static and dynamic fashion. Osseous changes as well as ligamentous competence may affect what is considered overall “normal” knee alignment. Muscle imbalances may affect the dynamic alignment of the knee. Assessment of knee alignment is important in three positions: weightless, standing, and with ambulation. A “weightless” or supine evaluation permits evaluation of ligament competence, range of motion, and patella tracking, while standing and ambulation will demonstrate any changes in the overall static and dynamic alignment of the knee.

Although caution should be used to describe “normal,” there is a general consensus regarding what should be considered normal knee alignment. The mechanical axis of the leg is a straight line drawn from the center of the hip joint to the center of the ankle joint. With neutral alignment, the mechanical axis will bisect the knee joint ( Fig. 9-1 ). Variability in femoral neck length and offset, femoral and tibial bowing, and femoral length can affect what is considered “normal.” Generally speaking, there is 6 or 7 degrees of valgus alignment between the long axis of the femoral and tibial shafts. The axis of the knee joint is approximately 3 degrees from perpendicular to the midline vertical axis of the body. This alignment places a larger distribution of body weight through the medial compartment of the knee.

KNEE ALIGNMENT IN TOTAL KNEE REPLACEMENT

In the infancy of TKA, there was much debate regarding how one should restore knee alignment, especially with the severe varus or valgus alignments that can be present in patients with osteoarthritis of the knee. Should a prosthesis attempt to mimic normal knee alignment and place more load through the medial compartment? Should other pathologic factors affect how the prosthetic knee alignment is placed? For example, should the alignment be changed to neutral in a patient who has lived his or her entire life with a bow-legged alignment?

Classic Alignment

Mechanical alignment in TKA has generally been divided into two camps: classic and anatomic alignment ( Fig. 9-2 ). John Insall supported the principles of classic alignment in which the prosthetic knee replacement should distribute the body loads symmetrically across the knee joint. This would avoid overloading one compartment. Hsu et al. confirmed that a neutral tibial resection followed by a 7-degree distal femoral resection provided equal stresses in the two compartments. An oblique tibial resection places uneven stresses on the proximal tibia, which can preclude premature clinical failure of the prosthesis.

In classic alignment, the prosthetic knee joint is placed parallel to the ground. The tibial resection is performed perpendicular to the long axis of the tibia. It is more reproducible to make a cut that is perpendicular to the long axis of the tibia, rather than an oblique cut. Furthermore, with the importance of tibial rotation, any rotational errors will be compounded with the requirement of a 3-degree medial and 10-degree posterior sloped cut. Further difficulty lies in the face of severe bone deformities as normal anatomic landmarks may be obscured.

The distal femoral resection that sets the alignment in full extension is generally performed at 5 to 6 degrees of valgus. The distal femoral resection is performed with an intramedullary or extramedullary guide that will provide the 5 to 6 degrees of valgus off of the anatomic axis of the femur. Newer techniques using computer navigation have avoided violation of the femoral intramedullary canal to perform this resection.

Classic alignment may change the “normal” alignment of some patients whose pre-arthritic alignment may have been outside what is considered “normal.” In order to avoid drastic changes, in the neutral and varus knees, cuts are performed at 5 to 6 degrees of valgus, while in the valgus knee, the distal femur is cut at 4 to 5 degrees. Furthermore, in certain situations such as the obese patient, it is important to avoid excessive valgus as this may cause soft tissue irritation between the two legs.

Traditionally, to obtain optimal information regarding the distal femoral resection angle, one would be able to accurately measure the distal femoral cut angle by assessing the mechanical and anatomic axes of the femur with a radiograph from the hip to the ankle, especially in light of any deformity. McGrory et al. determined that there was no difference in the postoperative mechanical alignment whether an arbitrary number of 5 degrees was chosen versus actual measurement of the distal femoral cut angle. Furthermore, the reliability of measurements made with these long-standing radiographs has been called into question.

Anatomic Alignment

The anatomic alignment was used by David Hungerford for use in cruciate retaining designs to maintain the anatomic position of the joint line. Femoral valgus was set at 9 to 10 degrees, while the proximal tibial resection was performed at 3 degrees of varus. The overall mechanical alignment of 6 degrees of valgus was maintained. In addition, the anatomic posterior slope of approximately 10 degrees is maintained. Hsu et al. confirmed that in a cruciate retaining design, this did provide even force distribution.

Alignment with Computer Navigation

Computer navigation has been introduced to improve overall knee alignment in both the coronal and sagittal plane, as well as the rotational alignment of implants. Numerous studies have evaluated the theoretical improvements in knee alignment provided with computer navigation. Most report a reduction in outliers with the use of computer navigation. Kim et al. report no improvement with the addition of computer navigation in a study composed of 100 patients. A meta-analysis of 33 studies (11 randomized) including 3423 patients concluded that computer navigation did reduce the risk of radiographic malalignment. It did lengthen the procedure by 23%. The clinical benefits have yet to be defined. Robot-assisted surgery brings computer navigation to the next level, where robotic assistance can perform the precision cuts. Bellemans et al. report on the radiographic alignment in 25 subjects where the alignment was within 1 degree in all except three cases. However, the authors have abandoned this technique due to the technical and operational difficulties. Further work is required to streamline this process.

CLASSIC THEORIES OF SURGICAL TECHNIQUE IN TOTAL KNEE ARTHROPLASTY

In the early days of TKA, the development of surgical technique and implant design led to two distinct philosophies: the gap balancing technique and the measured resection technique. The gap balancing technique was developed in conjunction with the implantation of the posterior cruciate sacrificing designs, while the measured resection technique was developed for use with cruciate retaining implant designs. As surgical techniques have improved, these two theories have gradually adopted principles from one another, and at this point, the distinctions have become blurred.

Gap Technique

The gap technique has traditionally been linked with implantation of posterior cruciate sacrificing designs. It can be used with cruciate retaining implants with a modular tibial component that allows to restore the joint line as well as careful balancing of the posterior cruciate ligament (PCL). The main principle of the gap technique is to equalize the flexion and extension gaps with soft tissue releases prior to performing the final bony cuts ( Fig. 9-3 ).

Initially, the development of the gap technique was dictated by the rather small inventory of implant sizes that were available. Often, an undersized implant had to be placed onto a larger femur, which led to overresection of the posterior condyles. To offset the risk of developing instability in flexion, surgeons were forced to resect less than 5 mm from the proximal tibia. This often times led to a flexion-extension mismatch with a very tight extension gap. This required re-resection of the distal femur. Furthermore, surgeons were further limited as the thickest polyethylene insert available at the time was 15 mm. Today, the gap technique is still used but with a full complement of implant sizing options; this prevents the tendency to overresect the posterior femoral condyles as well as permitting a more substantial resection of the proximal tibia at approximately 10 mm while maintaining a balanced flexion-extension gap.

Tibial Resection

A sequence of steps does not have to be rigidly followed with the gap technique. Traditionally speaking, the proximal tibia is resected first at about 10 mm from the least compromised plateau, while being perpendicular to the long axis of the tibia. Initially, there was concern regarding the structural integrity of the proximal tibia at this resection depth. This forced many surgeons to make very thin cuts on the proximal tibia, which led to the use of relatively thin polyethylene inserts. The high stresses that are placed on a thin polyethylene caused premature failure in the clinical setting. Further clinical experience with a proximal tibial cut of approximately 10 mm demonstrated no loss of structural integrity of the proximal tibia, while basic science data was able to support the clinical data ( Fig. 9-4 ).