Summary
Optimizing alignment in total knee arthroplasty requires an understanding of the assumptions of a chosen instru- mentation system. This understanding involves knowing the possible alignment errors of a system and knowing how the particular system leads the surgeon through the various steps of component placement. Current in- tramedullary and extramedullary instrumentation can assist in component placement; however, the surgeon must be aware of situations such as extra-articular defor- mities that can affect the final alignment. Additionally, computer-assisted knee navigation is available for routine use.These systems allow for more accurate positioning of jigs, the ability to correctly establish femoral component rotation, instantaneous feedback on overall alignment, and the ability to prevent implantation of malpositioned components. Overall, computer assistance results in decreased variability and the elimination of outliers. An armamentarium of alternative techniques must be kept in mind for use as secondary checks on the primary tech- nique and in situations of distorted or absent anatomical landmarks.
Introduction
Optimizing total knee arthroplasty alignment requires an understanding of the assumptions of a chosen instru- mentation system. This understanding involves knowing the possible alignment errors of a system and knowing how the particular system leads the surgeon through the various steps of component placement. Current in- tramedullary and extramedullary instrumentation can assist in component placement; however, the surgeon must be aware of situations such as extra-articular defor- mities that can affect the final alignment. An armamen- tarium of alternative techniques must be kept in mind for use as secondary checks on the primary technique and in situations of distorted or absent anatomical land- marks.
The History of Optimal Alignment
A consensus exists on most aspects of “normal” knee alignment. Such normal alignment involves issues of the proper mechanical axis and joint line orientation. More specifically, proper alignment at the knee can be charac- terized by two independent conditions:
1. The normal or prosthetic knee joint should be cen- tered on the mechanical axis of the lower extremity.
2. “Proper” orientation of the joint line should exist.
Few dispute that the single most important element to successful and long-lasting total knee arthroplasty is accurate alignment of the implants [1-7]
Historically,instrumentation systems have been grouped under two headings, referred to here as classical align- ment and anatomical alignment [8]. In the classical, most common variety, the theoretically correct goal is the es- tablishment of a joint line perpendicular to the reconsti- tuted mechanical axis. As a result, the proximal tibial cut is perpendicular to the overall tibial shaft axis; and since the distal femoral cut is perpendicular to the femoral por- tion of the mechanical axis, it is oriented at an angle β, approximately 6° relative to the femoral shaft axis (⊡ Fig. 25-1).
25 Optimizing Alignment
M. A. Rauh, W. M. Mihalko, K. A. Krackow
⊡ Fig. 25-1.Variation in alignment from the mechanical to the anatomical axes of the femur
them accurately on preoperative radiographs is helpful when it is necessary to judge the accuracy of instrument placement intraoperatively. Regardless of the ultimate depth or thickness of the distal femoral or proximal tib- ial cut, the angular orientation selected by the instru- mentation system should lead to a bone resection situa- tion that resembles what was drawn and was predicted on the planning films.
Historically,instrumented approaches to establishing proper varus or valgus orientation of the distal femoral cut involved several different techniques: (a) intra- medullary alignment rods,(b) extramedullary alignment rods that are meant to parallel the femoral shaft, and (c) extramedullary alignment that is intended to point toward the femoral head. The first two are obviously directed toward referencing the anatomical femoral shaft axis,and the third references the mechanical femoral axis.
Whichever axis one decides to reference,theoretically the same angular orientation of cuts should be determined by any one of the three techniques [8].
Each of these three approaches has its sources of er- ror, which should be understood along with their magni- tudes. One type of error relates to incorrect mediolateral positioning of an alignment instrument, either in the region of the knee or at the hip. Simple high school trigonometric calculations indicate that 1-in mediolater- al placement errors in location of a femoral head will lead to a 3°-4.5° error in the orientation of the cut. At the hip, medial displacement leads to valgus change, whereas lateral displacement leads to varus change.
At the knee itself, the possibility also exists for mediolateral error, either at the entry point for an in- tramedullary rod drill hole or as other types of instru- ments are applied to the end of the femur. Whether the alignment guide points to the femoral head or is directed up the femoral shaft, mediolateral error in placement of the instrument at the knee leads to a similar varus or val- gus error. At the distal femur, lateral displacement leads to valgus change and medial displacement to varus posi- tioning. Mediolateral displacement distally has been minimized as a source of error. However, even at 1° or 2°
it is still a possible source that can add to other causes.
Sagittal plane alignment must also be considered with in- tramedullary femoral instrumentation. Minimal dis- placement can lead to malalignment of femoral compo- nents,and proper orientation of the femoral starting hole can influence the flexion of the ultimate component [12].
Intramedullary femoral instrumentation for estab- lishing the varus and valgus orientation of the femoral cut is clearly the most popular. With the use of an in- tramedullary alignment rod, it may be argued that un- certainty about the position of the proximal tip of the rod is minimized. This is certainly true as long as (a) the rod
er,as the resting point of the proximal tip of the rod comes to be more distal, closer to the knee, i.e., if the rod does not pass easily up the shaft, then the potential for error is significant. The possibility of problems from a flexible rod has to be considered a source of very significant, essentially uncontrollable error.
Originally, nearly all proximal tibial resections were performed perpendicular to the tibial shaft axis,and a rel- atively short cutting instrument was used on the anterior tibial crest. An obvious source of error was the potential of a proximal bow of the tibial cortex. It was hoped this would be offset, however, by a tendency to hold the over- all axis of the tibia in a vertical position and to use this overall orientation to keep the cuts perpendicular.
More specific tibial alignment guides have been in- troduced. Two available types focus on (a) extra- medullary alignment from the center of the knee to the ankle and (b) an intramedullary system. Just as with the femoral case, one is specifically cautioned concerning the accuracy of mediolateral placement of any such guides.
Erroneous lateral positioning at the ankle moves the in- strument toward a varus error, as would inappropriate medial positioning at the proximal surface of the tibia.
The magnitude of errors for a given amount of mediolat- eral displacement is similar in the tibia – actually slight- ly higher, because the bone is usually shorter.
One must also be careful with the use of in- tramedullary rods at the tibia. The obvious shortcomings of intramedullary instrumentation can be even more fre- quent and less obvious on the tibial side. Intramedullary alignment instrumentation will clearly direct proper placement when a non-flexible rod traverses the entire length of the tibia. It is not uncommon, however, for a medial convexity not only to block the full transverse of the IM rod but also to lead the tip into a medial direction which directs a fixed cutting jig into valgus.
Another factor interfering with full placement of the rod is the generally narrower tibial canal. The tibial in- tramedullary diameter seems to be approximately 2 mm less than that of the femur. However, one must resist the temptation to address this problem by simply using a smaller tibial alignment rod, as any flexibility of the rod will be equally problematic.
Accurate placement of tibial alignment instruments is also a concern. The specific cutting guides themselves need to be placed in the proper position. However, play in the instruments, and especially inadvertent movement of fixation pins, can lead to very realistic errors of 1°-4°.
If you consider this statement along with the similar one from the femoral section earlier, it is possible to see how surgeons using even sophisticated; modern total knee arthroplasty instruments can make overall varus or val- gus alignment errors exceeding 5°.
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Whenever one talks about 1° and 2° alignment or jig placement errors a typical comment arises: “But how accurately can we perform cuts or make any of these placements anyway?” The implication is that we do not need to worry about sources of error which are on the order of 1°-2°. This line of reasoning or casualness is fallacious. We have multiple steps and multiple sites or sources of 1°-2°, even 3°-4° possible errors. The combina- tion of several sources of error all “lining up” in the same direction may be quite rare. Nonetheless, who wants to have even 1-100 cases with 5°-7° of varus or valgus error?
A total knee replacement with a tibiofemoral angulation of 11° instead of 6° is quite obviously crooked. A varus error of 5°-6° may not be cosmetically obvious; mechan- ically, however, it may be even worse.
The topic of femoral component rotation introduces yet another aspect of knee arthroplasty that must be con- sidered in the quest for optimal alignment. However, prior to focusing on the mechanics of establishing rota- tional alignment with total knee instruments, the surgeon should keep foremost in mind the patient’s preoperative ro- tational characteristics. How do the patient’s clinical, anatomical landmarks appear preoperatively in a standing position, during gait, and especially on the examining or operating table? In particular, does the knee appear, in general, to be internally or externally rotated? What is the preoperative position of the tibial tubercle, the inter- malleolar axis,and the long axis of the foot? Ideally,the sur- geon ought to know how these all line up before disrupting the bony and soft-tissue relationships with surgery.
The crude instrumentation of the early 1970s made no clear-cut provision for femoral rotation, and, historically, surgeons generally aligned the cutting instrument with the “horizon of the room.” Later attempts involved ori- enting the anterior and posterior femoral cuts in a rota- tional sense so that the flexion space created was approx- imately rectangular. Preferred techniques developed which involved specific referencing from particular as- pects of the bony architecture of the distal femur. Here, in general terms, an attempt is made to reconstruct normal anatomy and at the same time to adjust safely for han- dling of the tibial cut.
In 1980, the Howmedica Universal Total Knee Instru- mentation system was introduced, which established femoral component rotation “anatomically” by keying from or referencing to the posterior condylar axis and di- recting the rotation of the posterior aspects of the femoral component to lie parallel to the original position of the patient’s posterior condyles.
Toward the mid 1980s, surgeons preferring to make proximal tibial cuts perpendicular to the overall tibial shaft axis were led by the analyses of John Moreland [2] to consider “externally rotating”the femoral component,that is, externally rotating relative to the posterior condylar axis line.These analyses recognized that for the knee in ex-
tension,the non-anatomical state of the classical cut made on the proximal tibia is offset by a corresponding and op- posite non-anatomical resection for the distal femur. In other words,a perpendicular proximal tibial cut can be ex- pected to remove more lateral tibia than medial tibia, and this is balanced in extension by the fact that the distal femoral cut removes greater bone medially than laterally.
(An assumption is implicit that the case example is free of bony deformity.) The overall orientation of the femoral shaft and tibial shaft axis is as desired,and the variation of the prosthetic joint line orientation from anatomically normal has been deemed acceptable and desirable by a substantial proportion of arthroplasty surgeons.
In general, it can be said that the femoral component externally rotated away from the posterior condylar axis may be seen to more closely parallel the epicondylar axis.
The rotational positioning of the femoral component to parallel the “epicondylar axis”has become a stated goal of many. Conceptually attempting to parallel or in any way reference the posterior condylar axis has always invited uncertainty in the presence of differential cartilage and/or bony wear. An instrument purporting to parallel the posterior condylar axis when there is common pos- teromedial wear rotates to some degree externally. In a valgus knee with very obvious wear on the posterior lateral condyle and also possibly an a priori hypoplastic condyle as part of the deformity, such a jig is directed to a position of internal rotation.
These considerations invited some additional appeal to referencing the epicondylar axis. Furthermore, cuts which parallel the epicondylar axis may naturally be well-behaved in providing proper soft-tissue balance and ligamentous stability, since the epicondyles are the origins of the collateral ligaments.
The introduction of the anteroposterior axis of Whiteside [13] added yet another axis to assist in the rotational alignment of femoral components.The AP axis is drawn as a line connecting the deepest point of the trochlear groove to a midpoint at the region of the poste- rior extent of the posterior intracondylar region. For those of us who have tried to draw this axis, it is usually obvious and seemingly reproducible. There is, however, a question of what to do with the patient whose trochlear groove is quite displaced due to patellofemoral joint pathology. In this situation and others, it really does appear that using the specifically termed Whiteside axis is a good bit like “eyeballing” the overall neutral rotation of the distal femoral bone. One has the concern of how specifically to align the instrument with this rotation, as most of the instrumentation we are putting onto the end of the femur obscures the view of the femoral bone, and hence the impression of where the axis is.
Tibial rotational landmarks, references for establish- ing tibial component rotation, are less distinct than what is present at the femur. At the tibia, however, similarly
following consistencies and limitations should be appre- ciated:
1. The overall appearance of the proximal joint surface of the tibia can be considered. In particular, a mid- transverse axis and/or a posterior cortical axis can be established. Bone loss, osteophyte formation, and general anatomical variability must be taken into ac- count. Furthermore, the normal configuration of the posterior cortex extending further posteriorly on the medial than on the lateral side should be noted.
Specifically, tibial fixation instruments which are de- signed to “key off ” the posterior tibial cortex must be regarded with very strong suspicion. This normal configuration dictates that, in the absence of defor- mation from asymmetric bone loss or residual osteo- phytes, alignment parallel to the posterior tibial cortical “axis” will lead to abnormal tibial component positioning. The component tends to be rotated in- ternally while the tibial bone itself is externally rotat- ed. In this situation the tibial tubercle is lateralized and patellar tracking is destabilized. External rota- tion of the foot and ankle may be clinically apparent.
2. The next readily available anatomical structure is the patellotibial tuberosity, which typically lies lateral to an anterior midline. Generally, it would be inap- propriate to aim a tibial plateau so that its neutral rotation axis pointed straight to the middle of the tibial tubercle.
3. Another anatomical landmark is the intermalleolar axis.
4. Still another landmark is the overall “long axis”of the foot as it is held up into neutral position of plantar flexion/dorsiflexion. These last two points, i.e., the in- termalleolar axis and the “long axis” of the foot, are moderately variable – as much as 15°-20° or even more – in a significant number of people. Because of these aspects of anatomical and rotational variability, it is wise to clinically assess these landmarks preopera- tively. One needs to know the starting point.
5. Another consideration for referencing tibial rotation might be the position established by the soft tissues.
This technique involves acceptance of the tibial rota- tion defined by the soft-tissue attachments. By this method, preparation of some or most of the joint sur- faces and also essentially all of the soft-tissue release have been performed, after which rotation of the tib- ia with regard to the femur is guided by soft tissues.
The neutral position of the tibia is marked to parallel that of the femur in extension when longitudinal ten- sion is applied across the joint. The neutral anterior- posterior rotation position of the tibia is taken to be the natural resting position, which is achieved during this tension-stress testing.
of femoral and tibial components [14]. However, the systematic approach to analyzing and correcting the deformity has allowed surgeons to arrive at a generally acceptable method of knee arthroplasty. Unfortunately, given the vast array of possibilities, true reliability is difficult to obtain.
Current Methods of Optimizing Alignment
Traditionally, surgeons performing total knee arthro- plasty have been concerned with optimizing the axial alignment of the lower extremity, i.e., varus and valgus.
However, as the influence of alignment on implant survival [1-7] became known, a greater consideration to rotational and medial-lateral positioning of components developed. In an effort to improve overall alignment and to minimize situations of gross malalignment, the senior authors developed a system for computer-assisted total knee arthroplasty [15].The current method of optimizing alignment involves utilization of the Knee Track Module (Stryker Navigation Systems, Stryker Howmedica Osteonics,Allendale, NJ) developed principally by the se- nior authors (K.A.K and W.M.M.)
An infrared sensor array located at the side of the operating room table is used by this system to localize emitters placed at specific locations on the lower extrem- ity of the patient.A tracking pin is placed into the patient’s ipsilateral iliac crest for the purpose of identifying the center of the femoral head. After the surgeon’s choice of incision, the distal femur and proximal lateral tibial bony prominences are identified and similar tracking pins are placed. With the infrared emitters placed onto the track- ing pins, the lower extremity can be manipulated in a cir- cular fashion about the hip. Using an iterative Gaussian algorithm to solve the true non-linear system, the center of the femoral head can be identified by the computer.
The center of the femoral head is calculated via a subrou- tine that assumes the head of the femur is spherical. Two hundred and fifty data points are collected in 20 s as the surgeon moves the lower extremity about the hip joint in a gentle spherical arc.The method of least squares is used to find the best-fit sphere given the data points collected.
The center estimate is then determined using an iterative Gauss-Newton algorithm to solve the true non-linear sys- tem.This algorithm runs through 200 iterations to assure convergence. Finally, the center point is transformed into the femoral reference frame. Using the pointing device, the geometry of the distal femur is digitized. One must identify the medial and lateral epicondyles, the center of the knee, and the anteroposterior axis of Whiteside, as well as the condylar surfaces.
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Likewise, the geometry of the proximal tibia is digi- tized as the surgeon identifies the sulcus between the tibial spines, the anteroposterior midpoint, and the medi- al and lateral malleoli, along with the center of the ankle.
Given all the defined points, the tibiofemoral and me- chanical axes can be determined before any bone cuts have been made. This determination then allows accurate de- piction of any correction needed to properly align the com- ponents. Additionally, this navigation system is able to identify intraoperatively the real-time relative position of the tibia in respect to the femur. That is, one is able to pre- cisely determine tibiofemoral angle, the flexion-extension position,and the amount of relative internal or external ro- tation of the tibia with respect to the femur. Also, changes in the relative tibiofemoral compression-distraction and mediolateral or anteroposterior displacements can be de- termined by calculating the change in the directional vec- tor from the tibial to the femoral reference frames.
The potential for error still exists with this system;
however, it ranges from 0.1 to 1 mm for each of the three coordinates [14], which is considerably less than that ob- tained from any other system available. Armed with this knowledge, the operating surgeon is able to utilize jigs from any specific instrument system to make a more accurate cut. Upon final verification of cuts, a more accu- rate alignment of the lower extremity can be obtained than previously without the use of this device. Addition- ally, knowledge of flexion-extension gaps at all ranges of knee flexion allow for more accurate ligament balancing techniques.
The ability to optimize the axial, rotational, and medial-lateral translational alignment with the assistance of computerized navigational assistance should provide a better aligned and balanced extremity. We anticipate that yet more accurate alignment will result in even longer life spans of given prostheses.
Computer-assisted knee navigation is available for routine use. It allows for accurate positioning of jigs, the
ability to correctly establish femoral component rotation, instantaneous feedback on overall alignment,and the abil- ity to prevent implantation of malpositioned components.
Overall,computer assistance results in decreased variabil- ity and the elimination of outliers (⊡ Fig. 25-2) [12, 16].
Given the successes of the first series of cases [17], potential clinical benefits would include faster rehabilita- tion and better range of motion. Additionally, the advent of modern computers and the development of procedure- specific software have assisted in the process of opti- mizing alignment and hopefully will result in improved long-term outcomes.
Acknowledgements. We appreciate the kind assistance we received in producing this chapter and extend our thanks to Dr. S. Munjal and Dr. M.J. Phillips, State University of New York at Buffalo, Kaleida Health, Buffalo General Hospital, Buffalo, NY.
References
1. Maestro A et al (1998) Influence of intramedullary versus extramedullary alignment guides on final total knee arthroplasty component position.
J Arthroplasty 13:552-558
2. Moreland JR (1988) Mechanisms of failure in total knee arthroplasty. Clin Orthop 226:49-64
3. Lotke P et al (1977) Influence of positioning of prosthesis in total knee replacement. J Bone Joint Surg [Am] 59:77-79
4. Windsor RE et al (1989) Mechanisms of failure of the femoral and tibial components in total knee arthroplasty. Clin Orthop Rel Res 248:15-20 5. Mont MA et al (1997) Intramedullary goniometer can improve alignment
in knee arthroplasty surgery. J Arthroplasty 12:332-336
6. Delp SL et al (1998) Computer-assisted knee replacement. Clin Orthop 354:49-56
7. Ritter M et al (1994) Postoperative alignment of total knee replacement.
Its effect on survival. Clin Orthop 299:153-156
8. Krackow KA (1990) The technique of total knee arthroplasty. Mosby, St. Louis
9. Chao E et al (1994) Biomechanics of malalignment. Orthop Clin North Am 25:379-386
10. Moreland JR et al (1987) Radiographic analysis of the axial alignment of the lower extremity. J Bone Joint Surg [Am] 69:745-749
11. Kettelkamp D et al (1972) A method for quantitative analysis of medial and lateral compression forces at the knee during standing. Clin Orthop 83:202-213
12. Mihalko WM et al (2004) Intramedullary and computer navigational femoral alignment in TKA. Trans. American Academy of Orthopedic Surgeons, San Francisco
13. Whiteside LA et al (1995) The anteroposterior axis for femoral rotational alignment in valgus total knee arthroplasty. Clin Orthop 321:168-172 14. Krackow KA et al (2003) Computer-assisted total knee arthroplasty:
Navigation in TKA. Orthopedics 26:1017-1023
15. Krackow KA et al (1999) A new technique for determining proper me- chanical axis alignment during total knee arthroplasty: progress toward computer-assisted TKA. Orthopedics 22:698-702
16. Mihalko WM et al (2004) Extramedullary, intramedullary and CAS tibial alignment techniques for TKA. Trans. American Academy of Orthopedic Surgeons, San Francisco
17. Phillips MJ et al (2003) Computer-assisted total knee replacement - results of the first 90 cases using the Stryker navigation system. In: Proceedings of conference on Computer-assisted orthopaedic surgery. Marbella, Spain
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Alignment (degrees - valgus/ + varus) Navigated
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⊡ Fig. 25-2.Postoperative radiographic alignment in patients undergo- ing TKA
