➢ Posterior tibial slope should be measured on a long lateral or an expanded lateral radiograph.
➢ Posterior tibial slope decreases the quadriceps force needed to exert knee extension moment.
➢ Posterior tibial slope parallel to natural tibial slope minimizes tibial component subsidence.
➢ Posterior tibial slope should be increased rather than releasing the posterior cruciate ligament (PCL) to restore normal kinematics in a knee that is tight in flexion.
➢ Larger tibial slope widens the flexion gap in posterior stabilized total knee replacement.
Posterior tibial slope, which is defined as the posterior inclination of the tibial plateau1, is an important feature in patients undergoing total knee arthroplasty as it affects the kinematics of the knee joint. It influences the stability of cruciate-retaining and posterior-stabilized knees in both the coronal and sagittal planes and also impacts the flexion angle of the knee (Table I). Reflecting its importance, the posterior tibial slope was adopted by the American Knee Society for the radiographic evaluation of knee arthroplasty and is commonly reported in studies involving the sagittal alignment of the tibial component in patients managed with knee arthroplasty2.
There is substantial inter-individual variation in the anatomical posterior slope of the knee, and this variance is also related to ethnicity and sex3. De Boer et al. measured the posterior slope of the medial tibial plateau in 105 human cadaveric tibiae and found substantial inter-individual variation3. The mean posterior tibial slope was 8.4°, the smallest angle was −3° (anterior slope), and the largest angle was 16°.
In the present review, we describe the measurement of posterior tibial slope and its effects on knee stability as well as the stresses on tibial subchondral bone and the cruciate and collateral ligaments, its relationship with the flexion angle and wear following total knee replacement, and its influence on extensor mechanism forces.
Measurement of Posterior Tibial Slope
As posterior tibial slope can affect outcomes and knee kinematics following procedures such as high tibial osteotomy as well as unicompartmental and total knee replacement, an exact measurement of posterior tibial slope is important. Although computerized tomography (CT) scans, magnetic resonance imaging (MRI), and radiographs have been used for the measurement of posterior slope, there appears to be little consensus on the ideal imaging and anatomic reference for measuring posterior tibial slope4. On CT and MRI scans, it is easy to discriminate between the lateral and medial tibial plateaus; however, only the proximal part of the tibia is observed and the determination of the longitudinal axis is not possible on most routine scans5. Posterior tibial slope is measured routinely on lateral radiographs; however, it is difficult to make a separate assessment of the medial and lateral plateaus5. Some authors consider conventional lateral radiographs to be inferior for determining tibial slope because of the difficulty of achieving a perfect lateral alignment6. On a true lateral radiograph, posterior tibial slope is defined as an angle between the tangent of the medial and lateral plateaus and a line perpendicular to the longitudinal mechanical axis (Fig. 1)7.
Brazier et al., with use of true lateral radiographs of the leg that showed the entire length of the tibia with exact superimposition of femoral condyles, compared six methods of measuring the tibial slope8. They measured the angle between the tangent to the medial tibial plateau and the perpendicular to the tibial proximal anatomical axis, tibial shaft anatomical axis, posterior tibial cortex, anterior tibial cortex, fibular shaft axis, and fibular proximal anatomic axis. The values obtained with the tibial shaft anatomic axis were used as a reference. The investigators found that posterior tibial slope values obtained with use of the tibial proximal anatomical axis and posterior tibial cortex strongly correlated with those obtained with use of the tibial shaft anatomical axis. These values were not influenced by age, sex, height, or weight. The predicted slope of the contralateral tibia after measurement of one side was found to be unreliable. Faschingbauer et al. analyzed the relationship between posterior slope measurements on short and long lateral knee radiographs and found that the measurement was more accurate on long lateral knee radiographs7. They found that short lateral radiographs overestimated posterior tibial slope by an average of 3°. The measurement of posterior tibial slope on expanded lateral radiographs showing 20 cm of the tibia correlated strongly with that on long lateral knee radiographs. As long radiographs are not always possible, the use of expanded lateral radiographs is recommended for the estimation of posterior tibial slope.
Most investigators use the medial tibial plateau for the measurement of posterior tibial slope because it is the major load-bearing compartment of the two plateaus and its geometry suits the requirements for obtaining an accurate measurement (that is, it is relatively flat, with little concavity, and provides proper landmarks for measurements3). Some studies have shown that there is a difference in posterior slope when using the medial and lateral tibial plateaus as the reference2. On true lateral radiographs, when the two plateaus are superimposed, the measurement of posterior slope correlates strongly with measurements made using the medial plateau as the reference9.
Effect of Posterior Tibial Slope on Knee Stability
Radiographic studies have shown a linear relationship between posterior tibial slope and tibial translation during weight-bearing5. Increased posterior tibial slope increases anterior tibial translation, leading to increased strain on the anterior cruciate ligament. Greater anterior tibial translation secondary to increased posterior tibial slope affects anteroposterior stability of the knee1. Increased posterior tibial slope might be a risk factor for rupture of the anterior cruciate ligament3. Theoretically, changing the posterior tibial slope can help to compensate for deficient cruciate ligaments.
Excessive posterior tibial slope in patients undergoing total knee replacement may cause anteroposterior instability, leading to anterior subluxation of the tibial component, thereby increasing shear stresses on the posterior part of the tibial polyethylene and causing aseptic loosening10. In contrast, a decrease in posterior tibial slope leads to increased stresses on the weak anterior subchondral bone, thereby increasing the risk of component subsidence11. Decreased posterior slope also leads to limitation of flexion due to a tight flexion gap12.
Singerman et al. investigated the effect of posterior tibial slope on the strain in the posterior cruciate ligament (PCL) following total knee replacement13. PCL strain was measured in seven fresh frozen cadavers at posterior slopes of 5°, 8°, and 10°. Total knee replacement was performed with use of a Miller Galante (II) PCL-retaining design (Zimmer). The strain measured at 5° of posterior tibial slope was greater than the strain measured at both 8° and 10°. The investigators found that a decrease in posterior tibial slope led to an increase in the distance between the tibial and femoral PCL insertions, leading to increased strain. Excessive PCL strain due to reduced posterior tibial slope results in improper knee kinematics and reduced range of motion.
Effect of Posterior Tibial Slope on Contact Stresses and Ligaments
In a human knee, the tibial subchondral bone strength is greatest in the center of the plateau, which lies beneath the load-bearing area, with anterior bone being relatively weaker14. The strength of the medial tibial plateau is greater than that of the lateral tibial plateau14. Goldstein et al. showed that the strength of the subchondral bone decreases with distance from the articular surface15. Hofmann et al. reported 40% greater load-carrying capacity and 70% greater stiffness when the posterior tibial slope was parallel to the preoperative slope as compared with when it was perpendicular to the long axis of the tibia11. If the posterior tibial slope is perpendicular to the long axis, more of the anterior bone is resected, leaving residual bone that is weaker15.
Lee et al. used a three-dimensional (3D) finite-element model to evaluate the effect of 0°, 7°, and 10° of posterior tibial slope on contact forces and ligament stresses in knees undergoing posterior stabilized total knee replacement16. The contact stresses were distributed over a wide area in knees with a posterior tibial slope of 10° and narrow area in knees with a posterior tibial slope of 0°. Stresses on collateral ligaments were low in knees with a posterior tibial slope of 10° and highest in knees with a posterior tibial slope of 0°.
Jojima et al. used five fresh-frozen human knee specimens that were implanted with a Profix total knee replacement (Smith and Nephew) and investigated the effect of PCL release and posterior tibial slope on knee kinematics12. The investigators evaluated the effects of PCL release and posterior tibial slope in a normal knee, a knee with a total knee replacement, and a knee with a total knee replacement in which ligaments were made tight in flexion. They found that partial release of the PCL reduced tension only in the PCL, whereas increasing posterior tibial slope relaxed the collateral ligaments and PCL by decreasing the distance between the ligament attachments during flexion. The investigators found that increasing the posterior tibial slope increased varus-valgus laxity and rotational laxity more than PCL release did in the knee that was tight in flexion. PCL release corrected anteroposterior tightness only and had no effect on collateral ligament tightness. The investigators recommended increasing posterior tibial slope rather than PCL release in a knee that is tight in flexion as a PCL release only might not be enough to restore normal knee kinematics. However, cadaveric studies using a certain design of tibial tray are difficult to generalize as the results may be specific to that tray, insert geometry, and keel design.
Effect of Posterior Tibial Slope on Postoperative Flexion Angle After Total Knee Replacement
Postoperative range of motion is an important factor determining the success of a total knee replacement. Bellemans et al. analyzed twenty-one cadaver simulations of a PCL-retaining total knee replacement with use of a 3D computer model and showed that a 1° increase in posterior tibial slope would improve the flexion by 1.7°17. In each knee, the tibial component was consecutively implanted with 0°, 4°, and 7° of posterior slope. The average maximum flexion was 104° at 0° of posterior tibial slope, 112° at 4°, and 120° at 7°.
Walker and Garg used a computer-simulation model of the knee joint and a prosthesis and noted that posterior tibial slope influenced the flexion angle18. The maximum flexion angle increased by 30° with a posterior tibial slope of 10°, whereas it decreased by 25° with an anterior slope of 10°. Malviya et al., in a study of patients who had undergone cruciate-retaining total knee replacement, found a correlation between posterior tibial slope and the maximum flexion angle at twelve months19. Group 1 (thirty knees) had ≤5° of posterior tibial slope, Group 2 (thirty-three knees) had >5° but <8° of posterior tibial slope, and Group 3 (thirty-eight knees) had 8° to 10° of posterior tibial slope. The study showed an increase of 2.6° of flexion with each degree of tibial slope at twelve months following total knee replacement.
Singh et al., in a study of 167 patients who underwent 209 primary total knee replacements with use of either the Scorpio NRG (Non-Restrictive Geometry; Stryker) (n = 78) or the Zimmer LPS (Legacy Posterior Stabilized; Zimmer) (n = 131) found that recreation of anatomical tibial slope appeared to maximize the flexion angle and the range of movement in posterior-stabilized knee replacements20.
Surgeons should avoid excessive posterior tibial slope as it causes increased anteroposterior laxity. This consideration is important in the context of a total knee replacement in which the anterior cruciate ligament is resected and the only passive restraint to anterior displacement of the tibia is the posterior lip of the tibial insert17. This posterior lip may become deficient, leading to excessive articular wear due to anterior displacement of the tibial component. Wasielewski et al. found increased posterior articular wear in association with increased tibial slope in their retrieval study of tibial polyethylene inserts21. Hungerford and Kenna cautioned against excessive posterior tibial slope in cruciate-retaining total knee replacements as it can lead to resection of the PCL tibial attachment, making the knee even more unstable in flexion22.
Excessive posterior tibial slope has been shown to cause wear and deformation at the anterior aspect of the tibial post of posterior stabilized total knee replacements in retrieval studies23,24. The combination of femoral component flexion and increased posterior tibial slope causes impingement of the femoral cam on the anterior aspect of the tibial post, leading to increased stresses and anterior post wear and deformation. Essentially, this combination is effectively causing hyperextension at the femorotibial articulation.
Oka et al. investigated the effects of posterior tibial slope on soft-tissue balance in cruciate-retaining (n = 20) and posterior-stabilized (n = 20) total knee replacements25. All patients received the NexGen total knee replacement (Zimmer). All total knee replacements were performed with use of a measured resection technique. The investigators found that the posterior tibial slope had no influence on the joint component gap in knees with cruciate-retaining total knee replacements, whereas there was a flexion-extension gap mismatch with larger posterior tibial slope in knees with posterior-stabilized total knee replacements. The flexion gap was larger than the extension gap. An increase in posterior tibial slope increases the flexion gap that is already increased by PCL release in a posterior-stabilized total knee replacement. This combination can lead to flexion instability in a posterior-stabilized total knee replacement. Thus, surgeons should be aware that larger tibial slope is a factor contributing to a widening of the flexion-extension gap in patients managed with posterior-stabilized total knee replacement.
Massin and Gournay measured the potential effects of posterior condylar offset, posterior tibial slope, and condylar rollback on knee flexion with use of femoral and tibial templates of a knee arthroplasty26. The results were verified by real implantation performed on sawbones. The investigators found that, with posterior condylar offset maintained constant, a 5° reduction in tibial slope (as compared with normal slope) decreased knee flexion by 3° to 4° as a result of tibiofemoral impingement. A >5° increase of tibial slope compared with normal increased the range of flexion by 7° with templates and by 10° in sawbones.
Kansara and Markel investigated the effect of posterior tibial slope on the range of motion after total knee replacement27. Thirty-one consecutive patients had a posterior-stabilized total knee replacement (Scorpio; Stryker) with a posterior tibial slope of 0°, and thirty patients had total knee replacement with a posterior tibial slope of 5°. The investigators found no significant difference in the postoperative flexion or clinical outcome as measured with the Hospital for Special Surgery (HSS) functional score. An important finding from that study was that cutting the proximal part of the tibia with the intent of creating a posterior tibial slope of 0° caused an anterior slope in some patients. The investigators recommended aiming for a posterior tibial slope of 5° to eliminate the anterior slope variance.
Effect of Posterior Tibial Slope on Extensor Mechanism Forces
Posterior tibial slope is associated with a potential improvement of quadriceps extension force after total knee arthroplasty.
Ostermeier et al. investigated the relationship between posterior tibial slope and the quadriceps force required to exert an extension moment after cruciate-retaining total knee replacement28. The investigators used cadaveric knee specimens that were tested in a kinematic knee-simulator after the implantation of a total knee replacement with a mobile-bearing insert. The knee prosthesis permitted anteroposterior sliding and rotation of the mobile insert. The investigators measured quadriceps load as well as insert and tibial displacement after tibial baseplate implantation with 0° and 10° of posterior slope. A posterior tibial slope of 10° led to a decrease in the quadriceps force required to exert an extension moment as compared with a posterior tibial slope of 0°. At 30° of knee flexion there was no significant difference in quadriceps force, whereas at 60° of knee flexion the quadriceps force needed to exert the same extension moment was substantially reduced. This decreased quadriceps force was due to a posterior shift of the femur relative to the tibia, leading to an increase in the quadriceps moment arm and therefore to a decreased quadriceps force needed to exert an extension moment. This concept has been supported by other studies1.
Effect of Posterior Tibial Slope on Rollback
A tight flexion gap in cruciate-retaining total knee replacements is often addressed by release of the PCL fibers. A tight PCL results in anterior lift-off of the tibial component in flexion, thus limiting flexion29.
In the study by Zelle et al., numerical analysis demonstrated that increasing posterior tibial slope enhanced femoral rollback patterns during deep knee flexion although releasing the PCL resulted in paradoxical anterior movement of the femur30. Thus, it appears that if the total knee replacement is tight in flexion intraoperatively or the tibial trial insert is lifting off anteriorly, it is better to increase the tibial slope rather than to release the PCL.
Effect of Posterior Tibial Slope on Tibial Component Loading
Many studies have shown a relationship between posterior tibial slope and tibial component longevity21,31. Hofmann et al. showed that a posterior tibial slope that was parallel to the preoperative posterior tibial slope reduced the subsidence and failure rates of tibial components11.
Bai et al., using sawbones tibiae with posterior tibial slopes of −5°, 0°, 3°, 6°, and 9°, implanted a tibial component (Foundation Knee System; Encore) with bone cement10. With use of a standard and then a highly congruent polyethylene insert, the knee was loaded at 0° and 30° of flexion and the anteroposterior motion of the tibial component and compressive strains were measured. Anterior micromotion of the tibial polyethylene component increased for each increase in the posterior tibial slope. Increased posterior tibial slope decreased the anterior tibial compressive strains. Posterior tibial slope preserves a greater amount of the anterior bone and shifts the tibiofemoral contact point to the posterior part of the tibial plateau11,18. This posterior shift of the tibiofemoral contact point reduces the risk of anterior subsidence of the tibial component and also provides a mechanical advantage to the quadriceps tendon due to an increase in the lever arm.
There is no consensus on the optimal posterior tibial slope in patients undergoing total knee replacement. It has been suggested that the posterior tibial slope in such patients should be parallel to the preoperative anatomical posterior tibial slope32-34. There is evidence to suggest that a posterior tibial slope of >8° may lead to anterior tibial subluxation, increased wear of the posterior lip of the polyethylene component, and lack of rollback of the femur in flexion21,35. Some studies have shown increased anterior tibial subluxation, increased wear of the posterior lip of the polyethylene, and lack of rollback of the femur in flexion if the postoperative slope is >8°3. Some patients might have a preoperative slope of >8°, so in order to replicate this preoperative slope during knee replacement, it is impossible not to cut a slope of >8°.
Tibial slope therefore has an effect on the stability of the knee, its maximum flexion, the tension on the cruciate ligaments, and pressure on the cartilage and underlying bone4. Posterior tibial slope affects the stability of a natural knee and the stability and kinematics of the knee after a total knee replacement7. Specifically, it affects the stability in the coronal and sagittal planes, with increasing posterior tibial slope causing varus and valgus laxity, anteroposterior laxity, and rotational laxity12. Increased posterior tibial slope also leads to an increase in the range of motion by loosening of the posterior cruciate ligament35.
Source of Funding: No external funds were received in support of this study.
Investigation performed at the Exeter Knee Reconstruction Unit, Royal Devon and Exeter Hospital, Exeter, United Kingdom
Disclosure: None of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support of any aspect of this work. None of the authors, or their institution(s), have had any financial relationship, in the thirty-six months prior to submission of this work, with any entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. Also, no author has had any other relationships, or has engaged in any other activities, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article.
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