INSTRUCTIONAL REVIEW: KNEE
Coronal alignment in total knee replacement HISTORICAL REVIEW, CONTEMPORARY ANALYSIS, AND FUTURE DIRECTION
M. P. Abdel, S. Oussedik, S. Parratte, S. Lustig, F. S. Haddad From Mayo Clinic, Rochester, Minnesota, United States
Substantial healthcare resources have been devoted to computer navigation and patientspecific instrumentation systems that improve the reproducibility with which neutral mechanical alignment can be achieved following total knee replacement (TKR). This choice of alignment is based on the long-held tenet that the alignment of the limb post-operatively should be within 3° of a neutral mechanical axis. Several recent studies have demonstrated no significant difference in survivorship when comparing well aligned versus malaligned TKRs. Our aim was to review the anatomical alignment of the knee, the historical and contemporary data on a neutral mechanical axis in TKR, and the feasibility of kinematicallyaligned TKRs. Review of the literature suggests that a neutral mechanical axis remains the optimal guide to alignment. Cite this article: Bone Joint J 2014;96-B:857–62.
M. P. Abdel, MD, Assistant Professor of Orthopedic Surgery and Orthopedic Surgeon Mayo Clinic, Department of Orthopedic Surgery, 200 First Street SW, Rochester, Minnesota 55905, USA. S. Oussedik, FRCS(Orth), Orthopedic Surgeon University College London Hospital, Department of Trauma and Orthopaedics, 235 Euston Road, London NW1 2BU, UK. S. Parratte, MD, PhD, Orthopedic Surgeon Institute for Locomotion, Department of Orthopedic Surgery, Aix-Marseille University, 270 Boulevard Sainte Marguerite, BP 29, 13274 Marseille, France. S. Lustig, MD, PhD, Orthopedic Surgeon Service de Chirurgie Orthopédique, Centre AlbertTrillat, CHU de Lyon-Nord, Lyon, France. F. S. Haddad , BSc MCh(Orth), FRCS(Orth), FFSEM, Editor-inChief The Bone & Joint Journal, 22 Buckingham Street, London, WC2N 6ET, UK. Correspondence should be sent to Dr M. P. Abdel; e-mail: [email protected] ©2014 The British Editorial Society of Bone & Joint Surgery doi:10.1302/0301-620X.96B7. 33946 $2.00 Bone Joint J 2014;96-B:857–62.
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The number of primary total knee replacements (TKRs) carried out every year in the United States (US) is estimated to increase by 673% before 2030.1 In contrast with patients who undergo primary total hip replacement (THR), approximately 20% of those who undergo TKR are not satisfied with the outcome2,3: the causes of dissatisfaction remain elusive. TKR is a bony and soft-tissue procedure; much attention has been given to the alignment of the components, which is relatively easily quantifiable, particularly in the coronal plane. Recently, substantial healthcare resources have been devoted to the development and use of computer navigation and patient-specific instrumentation systems that achieve neutral mechanical alignment.4-6 This choice of alignment is based on the long-held tenet that postoperative alignment of the lower limb should be within 3° of a neutral mechanical axis.7-16 With modern implants and fixation techniques, some have debated describing alignment as a dichotomous variable (aligned or malaligned) on the basis of a mechanical axis goal of 0° (SD 3°).4,6,17-20 Others have supported the concept of a kinematicallyaligned TKR.21-23 Our aims were to review firstly, the relevant anatomy and alignment of the knee; secondly, the historical literature of neutral mechanical alignment in TKR; thirdly, the contemporary debate on coronal alignment in TKR and
fourthly, the evidence, or lack thereof, supporting the use of kinematic alignment.
Anatomy and alignment It is important, firstly, to clarify that alignment in the lower limb is referenced from a vertical midline through the pubic symphysis.24 The anatomical axes are lines drawn along the length of the intramedullary canals of the femur and tibia. The anatomical axes of the joint surfaces refer to lines drawn perpendicular to a line joining the most distal femoral or most proximal tibial points of the joint surfaces of either bone. The mechanical axis is a line drawn from the centre of the femoral head to the centre of the talus, and is commonly referred to as Maquet’s line.13,25 With this in mind, the anatomical alignment of the femoral joint surface is about 9° of valgus from the midline, whereas the anatomical alignment of the tibia is about 3° of varus from the midline. Typically, the mechanical alignment of the tibia is equivalent to the mechanical alignment of the limb and the tibial mechanical-anatomical (TMA) angle is 0°, or neutral. The mechanical alignment of the femoral joint surface is about 3° of valgus from the vertical midline and the femoral joint surface mechanical-anatomical (FMA) angle is about 6° of valgus. Considering these definitions, overall alignment can be described in two ways, either by the anatomical femoral-tibial (AFT) angle or the mechanical femoral-tibial 857
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(MFT) angle. The AFT angle is simply the difference between the anatomical alignment of the femoral joint surface (9° of valgus) and tibia (3° of varus), and is usually about 6° of valgus. Likewise, the MFT angle is the difference between the mechanical alignment of the femoral joint surface (3° of valgus) and tibia (3° of varus), resulting in 0° or neutral mechanical alignment. While the AFT angle can be estimated from short- or long-leg radiographs, accurate measurement of the MFT angle requires long-leg radiographs. Critics of older studies, which support the use of a ‘coronal safe zone’, often note that such investigations are limited by the use of short-leg radiographs.4,20,26-28 Peterson and Engh28 studied the anteroposterior (AP) radiographs of 50 knees and recorded that the mean difference between the tibiofemoral AFT angle on short- and long-leg radiographs was 1.4° (SD −3° to 5°; p < 0.001).20,28 However, some studies show a correlation between the anatomical and mechanical axes, supporting the use of short-leg radiographs.10,11,26,28,29 In our opinion, standing long-leg radiographs should be used as they do not require a surrogate mechanical axis to be determined from the anatomical axis. Historical review. It has long been suggested that restoration of a neutral mechanical axis improves durability following TKR. Data from many clinical, simulator, finite element, and retrieval studies have led to this belief.7-16,30-35 However, a closer analysis of these studies is required as many involved small numbers of patients, early designs of components, and/ or used short-leg radiographs.4,15,30,32-34,36 In 1977, Lotke and Ecker32 showed that good clinical results were associated with a geometric TKR anatomically positioned between 3° and 7° of valgus. Hood et al37 and Hvid and Nielsen34 defined the ideal range of anatomical alignment as between 2° and 12° of valgus, whereas Moreland et al27 recommended between 0° and 10°of valgus. Mallory, Smalley and Danyi38 recommended a target of ± 10° from the long-axis based on short-leg radiographs. In a clinical and cadaveric study, Bargren et al36 found that the Freeman–Swanson (ICLH) knee failed at lower compressive loads and had a higher rate of failure when aligned in varus. Insall et al31 believed that the mechanical axis should lie lateral to the centre of the knee producing valgus, whereas Townley39 believed that the mechanical axis should lie medial to the centre of the knee, in varus. Ritter et al15 showed that the posterior cruciate condylar TKR should be aligned in neutral or slight valgus (5° to 8° of anatomical valgus) for improved survival. All of the studies mentioned above used short-leg radiographs and older designs of components. Jeffery et al13 popularised the restoration of the mechanical axis to 0° (SD 3°) referenced from Maquet’s line with the use of long-leg radiographs. While a rather unique design of implant was used in their study, many subsequent finite model analyses and laboratory investigations including simulator studies and those in cadavers, have supported this target.8,9,12,16,30,35
A landmark article by Hsu et al30 in 1990 studied 120 normal subjects of various ages and both genders with fulllength weight-bearing radiographs of the lower limb and noted several intriguing findings. Foremost, they found that the angle formed by femoral and tibial mechanical axes was 1.2° of varus; thus it was difficult to rationalise the placement of a tibial component in 3° of varus. Secondly, they noted that the normal anatomical-mechanical angle of the femoral joint surface was between 4° and 5° depending on whether short- or long-leg radiographs were used. Finally, they found that with a single-leg stance, 75% of load passed through the medial tibial plateau.
Contemporary analysis The current understanding of the native, arthritic, and replaced knees has significantly improved. Combined with recent advances in technology, this has led to the further investigation of coronal alignment4,6,10,11,15,19,20 Several recent studies have reported no significant difference in survivorship when a traditionally held safe zone of 0° (SD 3°) was used to define aligned versus malaligned knees.4,6,19,20 In one of the most influential studies, Parratte et al4 retrospectively reviewed the clinical and radiological data of 398 cemented primary TKRs undertaken with one of three contemporary designs. All patients had pre- and postoperative full-length standing radiographs. They found that a post-operative mechanical axis of 0° (SD 3°) did not improve the rate of survival 15 years post-operatively, and concluded that the description of alignment as a dichotomous variable (aligned vs malaligned) provided little value in regards to durability. However, they also stated that: until additional data can be generated to more accurately determine the ideal post-operative limb alignment in individual patients, a neutral mechanical axis remains a reasonable target and should be considered as the standard for comparison if other alignment targets are introduced.4 Bonner et al19 subsequently completed a similar study that described knees as either aligned, with a mechanical axis of 0° (SD 3°), or malaligned, with a mechanical axis deviated from neutral by > 3°, at 15 years. Similar to Parratte et al,4 the authors found that the relationship between coronal alignment and survivorship was weak.19 In a smaller study, Matziolis et al6 found no difference in survival or outcome between aligned TKRs and a subset of varus outliers. However, only 30 ‘malaligned’ TKRs were examined, and there were no revisions in either group. The authors emphasised that “correct component alignment should be intended in every case”.6 Similarly, Morgan et al20 described 197 TKRs, which were divided into three groups based on the AFTs: neutral (4° to 9° of valgus), valgus (> 9.1° of valgus), and varus (< 3.9° of valgus), and found no difference in survivorship when they compared anatomical alignment based on longleg radiographs. On the other hand, Berend et al7 reported a statistically increased rate of failure of tibial components positioned in THE BONE & JOINT JOURNAL
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> 3.9° of varus. This was accentuated statistically if the body mass index was > 33 kg/m2. In 2009, Fang et al11 expanded on this data set with six further years of followup and examined all, not just tibial-sided, failures. They found that in 6070 TKRs, the best survival was in those with an overall anatomical alignment of between 2.4° and 7.2° of valgus. However, the difference in the rate of revision between the well-aligned and poorly aligned groups was only 1%. They used short-leg radiographs to assess alignment. More recently, Ritter et al14 looked at this cohort to record survivorship further as it relates to the anatomical alignment of each component, the overall anatomical alignment, and neutral alignment with both components malpositioned. They found an increased rate of failure in those with a femoral component in > 8° of anatomical valgus, in those with a varus tibial component relative to the tibial axis, or when one component was introduced in such a way as to compensate for malalignment of the other component, resulting in neutral alignment. Finally, Collier et al40 reported a significantly greater loss of thickness of polyethylene in the medial compartment when the limb was aligned in > 5° of varus. While the emphasis of this paper is not the clinical and functional outcomes related to alignment, it is important to briefly mention recent findings on the topic. While Magnussen et al41 found no difference in International Knee Society scores between TKRs which were in neutral versus varus mechanical alignment, two recent studies have noted improvements in the one year functional outcomes for those with coronal alignment within 3° of neutral.42,43 Choong et al42 found better International Knee Society and Short-Form-12 physical scores at six weeks, and at three, six and 12 months after surgery for those with a neutral mechanical axis of 0° (SD 3°). Similarly, Longstaff et al43 found that patients with aligned TKRs had improved Knee Society Scores one year post-operatively.
Kinematically-aligned TKR The concept of constitutional varus was popularised by Bellemans et al,17,18 who studied 250 asymptomatic adult volunteers between the ages of 20 and 27 years. They found that 32% of men and 17% of women had constitutional varus of their knees, with a natural mechanical alignment of > 3° varus. They suggested that restoration of neutral mechanical alignment in these patients might not be desirable. The concept of a single flexion axis for the knee is also becoming more widely recognised.44 However, how to best find this axis intra-operatively remains controversial. The single axis around which the tibia rotates is not captured in any of the traditional coronal, sagittal, or transverse planes. As such, a surrogate axis passing through the most medial and lateral portions of the epicondyles, the transepicondylar axis (TEA), has been proposed to represent the best approximation of the actual flexion-extension axis (FEA) of the knee.44,45 Using two- dimensional analysis, Churchill et al45 concluded there was a statistically insignificant VOL. 96-B, No. 7, JULY 2014
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difference of 3° (SD 1°) between the TEA and the true FEA. However, recent work by Eckhoff et al46 suggested that the TEA might be an unsatisfactory surrogate of the FEA because it lies anterior and superior to the FEA. Moreover, it has been suggested that the cylindrical axis, defined as a line equidistant from contact points on the medial and lateral condylar surfaces between 10° and 120° of flexion, forms angles more orthogonal to the mechanical axes of the thigh and leg than the TEA.3,36,44, 46-51 For similar reasons, Howell et al21-23 have promoted ‘kinematically-aligned’ TKRs with the goal of restoring the alignment to that which it would have been before the onset of arthritis, and avoiding release of the collateral ligaments. They reported on 214 kinematically-aligned, cruciateretaining TKRs at a mean follow-up of 31 months. The patients were divided into three groups: limbs in the neutral range (0°, SD 3°), varus (> 3°), or valgus alignment (> 3°). They found no catastrophic failures in the short-term, and concluded that in kinematically-aligned TKRs a “high risk for catastrophic failure is unfounded and should be of interest to surgeons committed to cutting the tibia perpendicular to the mechanical axis of the tibia”.22 However, we should be cautious for several reasons. Firstly, while malalignment may not cause ‘catastrophic’ failure in the very short-term (at a mean of 31 months), such data cannot be extrapolated to the mid- or long-term. Secondly, kinematically-aligned TKRs require the use of patient-specific instrumentation. Finally, these findings are yet to be widely reproduced. The popularity surrounding kinematically-aligned TKRs reflect concern with the high rate of dissatisfaction after TKR.2 While alignment certainly may contribute to these results, there are many other host, surgeon, and environmental factors that play a significant, but as yet undefined, role. From a conceptual standpoint, it is certainly intriguing to reproduce the anatomy of patients. However, as previously noted, the clinical and basic science data on which such an approach is based, is sparse. To our knowledge, there is one randomised controlled trial (RCT) by a group other than Howell et al3,21-23 Dossett et al3 compared kinematically-aligned TKRs with mechanically-aligned TKRs in a RCT of 82 patients. They found that the angle of the femoral component was a mean of 2.4° more valgus and the angle of the tibial component was a mean of 2.3° more varus than the mechanicallyaligned group.3 Moreover, they noted that at six months post-operatively, the mean Western Ontario and McMaster Universities Osteoarthritis Index score was 16 points better, the mean Oxford Knee Score was seven points better, the mean combined Knee Society Score was 25 points better and the mean range of flexion was 5.0° greater in the kinematically-aligned group.3 While no reason for the differences is presented, contributory factors might include the fact that only 66% of the randomised patients were analysed, the study was powered for alignment and not for clinical outcomes, and patient-specific instrumentation was
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used in the kinematically-aligned group. Their findings are in contrast to other reports.42,43 It should also be noted that hitherto, the focus for research and development has involved the assessment of coronal alignment of the lower limb in full extension. Activities of daily living require the lower limbs to transmit force throughout the range of movement. On flexion of a TKR, the coronal alignment varies with axial alignment, soft-tissue tension and the design of the components. If the goal of TKR is to reposition the flexion-extension axis in such a way as to minimise strain on the surrounding soft tissues, one can understand how the position of the components in all three planes will influence both function and survival. Although more difficult to achieve and measure reproducibly, the goals of alignment in the sagittal and axial planes remain poorly understood. While many authors have claimed that it is more difficult to obtain appropriate sagittal alignment than coronal alignment,52-54 few have evaluated the effects of sagittal alignment on function and survival. However, sagittal instability does occur due to malalignment.55,56 Kim et al53 found that flexion of the femoral component of > 3° or sagittal alignment of the tibial component of < 0° or a tibial slope of > 7° were risk factors for failure. Likewise, there is little reliable evidence of the effect of rotational alignment on survival because the techniques for measuring it intra- and post-operatively are often inaccurate, and the optimal rotational alignment has not been defined.6,53 However, axial rotation of the femoral component is crucial for obtaining well-balanced flexion gaps and tibiofemoral and patellofemoral congruency during flexion.57-59 In addition, rotational alignment is critical to the outcome of TKR.15,27,57,58 Kim et al53 reported that external rotation of the femoral component of < 2° or > 5° increased the rate of failure significantly. Furthermore, external rotation of the tibial component of < 2° or > 5° also increased the rate of failure significantly. The rotation of the components may have a greater role on the outcome after TKR than is measurable. Thus, the relationship between internal rotation of the components and patellar maltracking was well demonstrated by Berger et al60 using axial CT scans to evaluate rotational alignment. He recorded a direct correlation between internal rotation of the components and patellar maltracking. Mild combined internal rotation of between 1° and 4° caused lateral patellar tracking and patellar tilting, whereas moderate internal rotation of between 5° and 8° caused subluxation, and marked internal rotation of between 7° and 17° caused patellar dislocation or failure. Rotation of the tibia is also critical.61-63 If the tibial component is internally rotated relative to the cut surface of the tibial plateau, the tibia will be externally rotated relative to the femur. The tibial tubercle will then be lateralised and the Q angle increased, predisposing to lateral patellar subluxation.64 This occurs when the surgeon is trying to introduce a symmetrical tibial component that is larger than ideal. Several studies have
reported higher rates of revision and less favorable clinical results in patients with rotational malalignment of the tibial component.65,66 Finally, it is important to highlight that even with computer-assisted navigation, there is significant human error that does not allow for perfect reproducibility of resection of the bone or positioning of the components. In a metaanalysis of alignment in computer-assisted surgery, Mason et al67 found 9% tibiofemoral, 4.9% femoral component, and 4% tibial component mechanical outliers. Thus, aiming for a ‘slight degree’ of varus requires the surgeon to be willing to accept 3° of intended varus, in addition to a potential human error of 3° to 4° of varus, resulting in unacceptable alignment.26 While some surgeons have advocated a paradigm shift in defining optimal coronal alignment, neutral alignment and classic bone cuts, remain the gold standard. This is based on the fact that there is sufficient data advocating neutral mechanical alignment with approximately 5° to 7° of anatomical valgus, but only minimal support for other targets. Although the precision of surgery can be improved with technical advances, human error will remain. Neutral mechanical alignment should remain the ‘safe zone’, even though it is becoming apparent that alignment cannot be described as a dichotomous variable. The authors did not receive any outside funding or grants in support of their research for or preparation of this work. One of the authors (FSH), or a member of his immediate family, received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (Smith & Nephew) for products not discussed in this chapter. One of the authors (MPA) works in a department that receives institutional research support from multiple commercial entities (Biomet, DePuy, Stryker, and Zimmer), while another author (FSH) works in a department that receives institutional research support from multiple commercial entities (Biomet, DePuy, Smith & Nephew, Stryker, and Zimmer). The author or one or more of the authors have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article. In addition, benefits have been or will be directed to a research fund, foundation, educational institution, or other nonprofit organisation with which one or more of the authors are associated.
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36. Bargren JH, Blaha JD, Freeman MA. Alignment in total knee arthroplasty. Correlated biomechanical and clinical observations. Clin Orthop Relat Res 1983;173:178– 183.
60. Berger RA, Rubash HE, Seel MJ, Thompson WH, Crossett LS. Determining the rotational alignment of the femoral component in total knee arthroplasty using the epicondylar axis. Clin Orthop Relat Res 1993;286:40–47.
37. Hood RW, Vanni M, Insall JN. The correction of knee alignment in 225 consecutive total condylar knee replacements. Clin Orthop Relat Res 1981;160:94–105.
61. Briard JL, Hungerford DS. Patellofemoral instability in total knee arthroplasty. J Arthroplasty 1989;(4Suppl):S87-97.
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62. Merkow RL, Soudry M, Insall JN. Patellar dislocation following total knee replacement. J Bone Joint Surg [Am] 1985;67-A:1321–1327. 63. Nagamine R, Whiteside LA, White SE, McCarthy DS. Patellar tracking after total knee arthroplasty. The effect of tibial tray malrotation and articular surface configuration. Clin Orthop Relat Res 1994;304:262–271. 64. Berger RA, Crossett LS, Jacobs JJ, Rubash HE. Malrotation causing patellofemoral complications after total knee arthroplasty. Clin Orthop Relat Res 1998;356:144– 153.
65. Hofmann S, Romero J, Roth-Schiffl E, Albrecht T. [Rotational malalignment of the components may cause chronic pain or early failure in total knee arthroplasty]. Orthopade 2003;32:469-476.[Article in German]. 66. Incavo SJ, Wild JJ, Coughlin KM, Beynnon BD. Early revision for component malrotation in total knee arthroplasty. Clin Orthop Relat Res 2007;458:131–136. 67. Mason JB, Fehring TK, Estok R, Banel D, Fahrbach K. Meta-analysis of alignment outcomes in computer-assisted total knee arthroplasty surgery. J Arthroplasty 2007;22:1097–1106.
THE BONE & JOINT JOURNAL
Coronal alignment in total knee replacement HISTORICAL REVIEW, CONTEMPORARY ANALYSIS, AND FUTURE DIRECTION
M. P. Abdel, S. Oussedik, S. Parratte, S. Lustig, F. S. Haddad From Mayo Clinic, Rochester, Minnesota, United States
Substantial healthcare resources have been devoted to computer navigation and patientspecific instrumentation systems that improve the reproducibility with which neutral mechanical alignment can be achieved following total knee replacement (TKR). This choice of alignment is based on the long-held tenet that the alignment of the limb post-operatively should be within 3° of a neutral mechanical axis. Several recent studies have demonstrated no significant difference in survivorship when comparing well aligned versus malaligned TKRs. Our aim was to review the anatomical alignment of the knee, the historical and contemporary data on a neutral mechanical axis in TKR, and the feasibility of kinematicallyaligned TKRs. Review of the literature suggests that a neutral mechanical axis remains the optimal guide to alignment. Cite this article: Bone Joint J 2014;96-B:857–62.
M. P. Abdel, MD, Assistant Professor of Orthopedic Surgery and Orthopedic Surgeon Mayo Clinic, Department of Orthopedic Surgery, 200 First Street SW, Rochester, Minnesota 55905, USA. S. Oussedik, FRCS(Orth), Orthopedic Surgeon University College London Hospital, Department of Trauma and Orthopaedics, 235 Euston Road, London NW1 2BU, UK. S. Parratte, MD, PhD, Orthopedic Surgeon Institute for Locomotion, Department of Orthopedic Surgery, Aix-Marseille University, 270 Boulevard Sainte Marguerite, BP 29, 13274 Marseille, France. S. Lustig, MD, PhD, Orthopedic Surgeon Service de Chirurgie Orthopédique, Centre AlbertTrillat, CHU de Lyon-Nord, Lyon, France. F. S. Haddad , BSc MCh(Orth), FRCS(Orth), FFSEM, Editor-inChief The Bone & Joint Journal, 22 Buckingham Street, London, WC2N 6ET, UK. Correspondence should be sent to Dr M. P. Abdel; e-mail: [email protected] ©2014 The British Editorial Society of Bone & Joint Surgery doi:10.1302/0301-620X.96B7. 33946 $2.00 Bone Joint J 2014;96-B:857–62.
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The number of primary total knee replacements (TKRs) carried out every year in the United States (US) is estimated to increase by 673% before 2030.1 In contrast with patients who undergo primary total hip replacement (THR), approximately 20% of those who undergo TKR are not satisfied with the outcome2,3: the causes of dissatisfaction remain elusive. TKR is a bony and soft-tissue procedure; much attention has been given to the alignment of the components, which is relatively easily quantifiable, particularly in the coronal plane. Recently, substantial healthcare resources have been devoted to the development and use of computer navigation and patient-specific instrumentation systems that achieve neutral mechanical alignment.4-6 This choice of alignment is based on the long-held tenet that postoperative alignment of the lower limb should be within 3° of a neutral mechanical axis.7-16 With modern implants and fixation techniques, some have debated describing alignment as a dichotomous variable (aligned or malaligned) on the basis of a mechanical axis goal of 0° (SD 3°).4,6,17-20 Others have supported the concept of a kinematicallyaligned TKR.21-23 Our aims were to review firstly, the relevant anatomy and alignment of the knee; secondly, the historical literature of neutral mechanical alignment in TKR; thirdly, the contemporary debate on coronal alignment in TKR and
fourthly, the evidence, or lack thereof, supporting the use of kinematic alignment.
Anatomy and alignment It is important, firstly, to clarify that alignment in the lower limb is referenced from a vertical midline through the pubic symphysis.24 The anatomical axes are lines drawn along the length of the intramedullary canals of the femur and tibia. The anatomical axes of the joint surfaces refer to lines drawn perpendicular to a line joining the most distal femoral or most proximal tibial points of the joint surfaces of either bone. The mechanical axis is a line drawn from the centre of the femoral head to the centre of the talus, and is commonly referred to as Maquet’s line.13,25 With this in mind, the anatomical alignment of the femoral joint surface is about 9° of valgus from the midline, whereas the anatomical alignment of the tibia is about 3° of varus from the midline. Typically, the mechanical alignment of the tibia is equivalent to the mechanical alignment of the limb and the tibial mechanical-anatomical (TMA) angle is 0°, or neutral. The mechanical alignment of the femoral joint surface is about 3° of valgus from the vertical midline and the femoral joint surface mechanical-anatomical (FMA) angle is about 6° of valgus. Considering these definitions, overall alignment can be described in two ways, either by the anatomical femoral-tibial (AFT) angle or the mechanical femoral-tibial 857
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(MFT) angle. The AFT angle is simply the difference between the anatomical alignment of the femoral joint surface (9° of valgus) and tibia (3° of varus), and is usually about 6° of valgus. Likewise, the MFT angle is the difference between the mechanical alignment of the femoral joint surface (3° of valgus) and tibia (3° of varus), resulting in 0° or neutral mechanical alignment. While the AFT angle can be estimated from short- or long-leg radiographs, accurate measurement of the MFT angle requires long-leg radiographs. Critics of older studies, which support the use of a ‘coronal safe zone’, often note that such investigations are limited by the use of short-leg radiographs.4,20,26-28 Peterson and Engh28 studied the anteroposterior (AP) radiographs of 50 knees and recorded that the mean difference between the tibiofemoral AFT angle on short- and long-leg radiographs was 1.4° (SD −3° to 5°; p < 0.001).20,28 However, some studies show a correlation between the anatomical and mechanical axes, supporting the use of short-leg radiographs.10,11,26,28,29 In our opinion, standing long-leg radiographs should be used as they do not require a surrogate mechanical axis to be determined from the anatomical axis. Historical review. It has long been suggested that restoration of a neutral mechanical axis improves durability following TKR. Data from many clinical, simulator, finite element, and retrieval studies have led to this belief.7-16,30-35 However, a closer analysis of these studies is required as many involved small numbers of patients, early designs of components, and/ or used short-leg radiographs.4,15,30,32-34,36 In 1977, Lotke and Ecker32 showed that good clinical results were associated with a geometric TKR anatomically positioned between 3° and 7° of valgus. Hood et al37 and Hvid and Nielsen34 defined the ideal range of anatomical alignment as between 2° and 12° of valgus, whereas Moreland et al27 recommended between 0° and 10°of valgus. Mallory, Smalley and Danyi38 recommended a target of ± 10° from the long-axis based on short-leg radiographs. In a clinical and cadaveric study, Bargren et al36 found that the Freeman–Swanson (ICLH) knee failed at lower compressive loads and had a higher rate of failure when aligned in varus. Insall et al31 believed that the mechanical axis should lie lateral to the centre of the knee producing valgus, whereas Townley39 believed that the mechanical axis should lie medial to the centre of the knee, in varus. Ritter et al15 showed that the posterior cruciate condylar TKR should be aligned in neutral or slight valgus (5° to 8° of anatomical valgus) for improved survival. All of the studies mentioned above used short-leg radiographs and older designs of components. Jeffery et al13 popularised the restoration of the mechanical axis to 0° (SD 3°) referenced from Maquet’s line with the use of long-leg radiographs. While a rather unique design of implant was used in their study, many subsequent finite model analyses and laboratory investigations including simulator studies and those in cadavers, have supported this target.8,9,12,16,30,35
A landmark article by Hsu et al30 in 1990 studied 120 normal subjects of various ages and both genders with fulllength weight-bearing radiographs of the lower limb and noted several intriguing findings. Foremost, they found that the angle formed by femoral and tibial mechanical axes was 1.2° of varus; thus it was difficult to rationalise the placement of a tibial component in 3° of varus. Secondly, they noted that the normal anatomical-mechanical angle of the femoral joint surface was between 4° and 5° depending on whether short- or long-leg radiographs were used. Finally, they found that with a single-leg stance, 75% of load passed through the medial tibial plateau.
Contemporary analysis The current understanding of the native, arthritic, and replaced knees has significantly improved. Combined with recent advances in technology, this has led to the further investigation of coronal alignment4,6,10,11,15,19,20 Several recent studies have reported no significant difference in survivorship when a traditionally held safe zone of 0° (SD 3°) was used to define aligned versus malaligned knees.4,6,19,20 In one of the most influential studies, Parratte et al4 retrospectively reviewed the clinical and radiological data of 398 cemented primary TKRs undertaken with one of three contemporary designs. All patients had pre- and postoperative full-length standing radiographs. They found that a post-operative mechanical axis of 0° (SD 3°) did not improve the rate of survival 15 years post-operatively, and concluded that the description of alignment as a dichotomous variable (aligned vs malaligned) provided little value in regards to durability. However, they also stated that: until additional data can be generated to more accurately determine the ideal post-operative limb alignment in individual patients, a neutral mechanical axis remains a reasonable target and should be considered as the standard for comparison if other alignment targets are introduced.4 Bonner et al19 subsequently completed a similar study that described knees as either aligned, with a mechanical axis of 0° (SD 3°), or malaligned, with a mechanical axis deviated from neutral by > 3°, at 15 years. Similar to Parratte et al,4 the authors found that the relationship between coronal alignment and survivorship was weak.19 In a smaller study, Matziolis et al6 found no difference in survival or outcome between aligned TKRs and a subset of varus outliers. However, only 30 ‘malaligned’ TKRs were examined, and there were no revisions in either group. The authors emphasised that “correct component alignment should be intended in every case”.6 Similarly, Morgan et al20 described 197 TKRs, which were divided into three groups based on the AFTs: neutral (4° to 9° of valgus), valgus (> 9.1° of valgus), and varus (< 3.9° of valgus), and found no difference in survivorship when they compared anatomical alignment based on longleg radiographs. On the other hand, Berend et al7 reported a statistically increased rate of failure of tibial components positioned in THE BONE & JOINT JOURNAL
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> 3.9° of varus. This was accentuated statistically if the body mass index was > 33 kg/m2. In 2009, Fang et al11 expanded on this data set with six further years of followup and examined all, not just tibial-sided, failures. They found that in 6070 TKRs, the best survival was in those with an overall anatomical alignment of between 2.4° and 7.2° of valgus. However, the difference in the rate of revision between the well-aligned and poorly aligned groups was only 1%. They used short-leg radiographs to assess alignment. More recently, Ritter et al14 looked at this cohort to record survivorship further as it relates to the anatomical alignment of each component, the overall anatomical alignment, and neutral alignment with both components malpositioned. They found an increased rate of failure in those with a femoral component in > 8° of anatomical valgus, in those with a varus tibial component relative to the tibial axis, or when one component was introduced in such a way as to compensate for malalignment of the other component, resulting in neutral alignment. Finally, Collier et al40 reported a significantly greater loss of thickness of polyethylene in the medial compartment when the limb was aligned in > 5° of varus. While the emphasis of this paper is not the clinical and functional outcomes related to alignment, it is important to briefly mention recent findings on the topic. While Magnussen et al41 found no difference in International Knee Society scores between TKRs which were in neutral versus varus mechanical alignment, two recent studies have noted improvements in the one year functional outcomes for those with coronal alignment within 3° of neutral.42,43 Choong et al42 found better International Knee Society and Short-Form-12 physical scores at six weeks, and at three, six and 12 months after surgery for those with a neutral mechanical axis of 0° (SD 3°). Similarly, Longstaff et al43 found that patients with aligned TKRs had improved Knee Society Scores one year post-operatively.
Kinematically-aligned TKR The concept of constitutional varus was popularised by Bellemans et al,17,18 who studied 250 asymptomatic adult volunteers between the ages of 20 and 27 years. They found that 32% of men and 17% of women had constitutional varus of their knees, with a natural mechanical alignment of > 3° varus. They suggested that restoration of neutral mechanical alignment in these patients might not be desirable. The concept of a single flexion axis for the knee is also becoming more widely recognised.44 However, how to best find this axis intra-operatively remains controversial. The single axis around which the tibia rotates is not captured in any of the traditional coronal, sagittal, or transverse planes. As such, a surrogate axis passing through the most medial and lateral portions of the epicondyles, the transepicondylar axis (TEA), has been proposed to represent the best approximation of the actual flexion-extension axis (FEA) of the knee.44,45 Using two- dimensional analysis, Churchill et al45 concluded there was a statistically insignificant VOL. 96-B, No. 7, JULY 2014
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difference of 3° (SD 1°) between the TEA and the true FEA. However, recent work by Eckhoff et al46 suggested that the TEA might be an unsatisfactory surrogate of the FEA because it lies anterior and superior to the FEA. Moreover, it has been suggested that the cylindrical axis, defined as a line equidistant from contact points on the medial and lateral condylar surfaces between 10° and 120° of flexion, forms angles more orthogonal to the mechanical axes of the thigh and leg than the TEA.3,36,44, 46-51 For similar reasons, Howell et al21-23 have promoted ‘kinematically-aligned’ TKRs with the goal of restoring the alignment to that which it would have been before the onset of arthritis, and avoiding release of the collateral ligaments. They reported on 214 kinematically-aligned, cruciateretaining TKRs at a mean follow-up of 31 months. The patients were divided into three groups: limbs in the neutral range (0°, SD 3°), varus (> 3°), or valgus alignment (> 3°). They found no catastrophic failures in the short-term, and concluded that in kinematically-aligned TKRs a “high risk for catastrophic failure is unfounded and should be of interest to surgeons committed to cutting the tibia perpendicular to the mechanical axis of the tibia”.22 However, we should be cautious for several reasons. Firstly, while malalignment may not cause ‘catastrophic’ failure in the very short-term (at a mean of 31 months), such data cannot be extrapolated to the mid- or long-term. Secondly, kinematically-aligned TKRs require the use of patient-specific instrumentation. Finally, these findings are yet to be widely reproduced. The popularity surrounding kinematically-aligned TKRs reflect concern with the high rate of dissatisfaction after TKR.2 While alignment certainly may contribute to these results, there are many other host, surgeon, and environmental factors that play a significant, but as yet undefined, role. From a conceptual standpoint, it is certainly intriguing to reproduce the anatomy of patients. However, as previously noted, the clinical and basic science data on which such an approach is based, is sparse. To our knowledge, there is one randomised controlled trial (RCT) by a group other than Howell et al3,21-23 Dossett et al3 compared kinematically-aligned TKRs with mechanically-aligned TKRs in a RCT of 82 patients. They found that the angle of the femoral component was a mean of 2.4° more valgus and the angle of the tibial component was a mean of 2.3° more varus than the mechanicallyaligned group.3 Moreover, they noted that at six months post-operatively, the mean Western Ontario and McMaster Universities Osteoarthritis Index score was 16 points better, the mean Oxford Knee Score was seven points better, the mean combined Knee Society Score was 25 points better and the mean range of flexion was 5.0° greater in the kinematically-aligned group.3 While no reason for the differences is presented, contributory factors might include the fact that only 66% of the randomised patients were analysed, the study was powered for alignment and not for clinical outcomes, and patient-specific instrumentation was
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used in the kinematically-aligned group. Their findings are in contrast to other reports.42,43 It should also be noted that hitherto, the focus for research and development has involved the assessment of coronal alignment of the lower limb in full extension. Activities of daily living require the lower limbs to transmit force throughout the range of movement. On flexion of a TKR, the coronal alignment varies with axial alignment, soft-tissue tension and the design of the components. If the goal of TKR is to reposition the flexion-extension axis in such a way as to minimise strain on the surrounding soft tissues, one can understand how the position of the components in all three planes will influence both function and survival. Although more difficult to achieve and measure reproducibly, the goals of alignment in the sagittal and axial planes remain poorly understood. While many authors have claimed that it is more difficult to obtain appropriate sagittal alignment than coronal alignment,52-54 few have evaluated the effects of sagittal alignment on function and survival. However, sagittal instability does occur due to malalignment.55,56 Kim et al53 found that flexion of the femoral component of > 3° or sagittal alignment of the tibial component of < 0° or a tibial slope of > 7° were risk factors for failure. Likewise, there is little reliable evidence of the effect of rotational alignment on survival because the techniques for measuring it intra- and post-operatively are often inaccurate, and the optimal rotational alignment has not been defined.6,53 However, axial rotation of the femoral component is crucial for obtaining well-balanced flexion gaps and tibiofemoral and patellofemoral congruency during flexion.57-59 In addition, rotational alignment is critical to the outcome of TKR.15,27,57,58 Kim et al53 reported that external rotation of the femoral component of < 2° or > 5° increased the rate of failure significantly. Furthermore, external rotation of the tibial component of < 2° or > 5° also increased the rate of failure significantly. The rotation of the components may have a greater role on the outcome after TKR than is measurable. Thus, the relationship between internal rotation of the components and patellar maltracking was well demonstrated by Berger et al60 using axial CT scans to evaluate rotational alignment. He recorded a direct correlation between internal rotation of the components and patellar maltracking. Mild combined internal rotation of between 1° and 4° caused lateral patellar tracking and patellar tilting, whereas moderate internal rotation of between 5° and 8° caused subluxation, and marked internal rotation of between 7° and 17° caused patellar dislocation or failure. Rotation of the tibia is also critical.61-63 If the tibial component is internally rotated relative to the cut surface of the tibial plateau, the tibia will be externally rotated relative to the femur. The tibial tubercle will then be lateralised and the Q angle increased, predisposing to lateral patellar subluxation.64 This occurs when the surgeon is trying to introduce a symmetrical tibial component that is larger than ideal. Several studies have
reported higher rates of revision and less favorable clinical results in patients with rotational malalignment of the tibial component.65,66 Finally, it is important to highlight that even with computer-assisted navigation, there is significant human error that does not allow for perfect reproducibility of resection of the bone or positioning of the components. In a metaanalysis of alignment in computer-assisted surgery, Mason et al67 found 9% tibiofemoral, 4.9% femoral component, and 4% tibial component mechanical outliers. Thus, aiming for a ‘slight degree’ of varus requires the surgeon to be willing to accept 3° of intended varus, in addition to a potential human error of 3° to 4° of varus, resulting in unacceptable alignment.26 While some surgeons have advocated a paradigm shift in defining optimal coronal alignment, neutral alignment and classic bone cuts, remain the gold standard. This is based on the fact that there is sufficient data advocating neutral mechanical alignment with approximately 5° to 7° of anatomical valgus, but only minimal support for other targets. Although the precision of surgery can be improved with technical advances, human error will remain. Neutral mechanical alignment should remain the ‘safe zone’, even though it is becoming apparent that alignment cannot be described as a dichotomous variable. The authors did not receive any outside funding or grants in support of their research for or preparation of this work. One of the authors (FSH), or a member of his immediate family, received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (Smith & Nephew) for products not discussed in this chapter. One of the authors (MPA) works in a department that receives institutional research support from multiple commercial entities (Biomet, DePuy, Stryker, and Zimmer), while another author (FSH) works in a department that receives institutional research support from multiple commercial entities (Biomet, DePuy, Smith & Nephew, Stryker, and Zimmer). The author or one or more of the authors have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article. In addition, benefits have been or will be directed to a research fund, foundation, educational institution, or other nonprofit organisation with which one or more of the authors are associated.
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