Ligament releases are necessary for contemporary non-conforming femoral-tibial articulations.

Most total knee arthroplasty prostheses are designed to be non-conforming at the articulation between the femoral and tibial components. This design is chosen on the arthroplasty principle that “constraint causes loosening” and conforming surfaces have been considered constrained. To provide stability the ligaments are adjusted so that tension in the ligament can provide stability for the total knee replacement.

Ligament releases are NOT necessary for contemporary conforming femoral-tibial articulations.

Through the majority of the range of motion, the normal human knee is not stabilised by ligament tension. Rather, it is the geometrical conformity of the femur and tibia, especially on the medial side, that provides stability. The ligaments are present and ready to restrain the knee from excess varus-valgus or anterior-posterior loads. In a knee design that is congruent, ligaments may be left intact as in the normal knee, ready to provide restraint but not necessarily to provide stability except when excess loads are applied to the knee.

When designing and using the ADVANCE Medial Pivot total knee, the author has left ligaments in the toe-region of the stress-strain curve rather than releasing and tensioning the ligaments. Patient satisfaction survey data at routine follow-up visits for patients at 7–15 years after arthroplasty with this type of reconstruction indicate high satisfaction despite medial and lateral opening (on valgus and varus stress) that would be considered “mid-flexion instability” for non-conforming joints that require careful ligament releases and tensioning.

Most total knee prostheses are designed to have limited congruence between the femoral and tibial components to reduce constraint, based on the widely accepted principle that “constraint causes loosening”.

Studies of the normal knee, however, indicate that stability under axial load occurs mostly by the geometric conformity of the surfaces. When moving in the plane of flexion-extension, the ligaments contribute little to stability because the ligaments are in the “toe-region” of their force-displacement curve. When an “out-of-plane” load is applied (i.e., load outside the plane of flexion-extension), ligaments are “recruited” for stability by being stressed into the elastic portion of the curve to resist the load.

For the traditional total knee prosthesis, because of the lack of geometric congruity, the ligaments must provide all stability by being “balanced”, i.e. tensioned into the elastic portion of the force-displacement curve. Further, they must remain in that tensioned state indefinitely, with no stretching or migration of the implant.

The medial pivot knee design has a fully conforming medial “ball-in-socket” articulation that provides stability to the knee through the geometric conformity. Ligaments need not be tensioned into the elastic region of the force-displacement curve but can be left in the toe-region to be recruited for out-of-plane loads.

Clinical follow-up results in patients with a medial pivot prosthesis indicate that, based on Knee Society and WOMAC scores, patients report greater than 90% satisfaction with pain and function. Further, the most satisfied patients are those who, during physical examination, display medial and lateral opening that might be classified as “mid-flexion instability” for prostheses that depend on ligament tensioning for stability.

In replacing the human knee, we attempt to reproduce the stability of the normal knee so that the knee will feel as close to normal as possible to the patient. To answer the question, “Which features matter?” we must first examine the stability of the normal knee. Compliance and stiffness: Stability is measured as “force-displacement” behavior. That is, a force is applied to the knee and the relative motion is measured. Engineers refer to the curves generated by this type of experiment as “stiffness”. Because stiffness is not a term that orthopaedists like to hear when referring to a knee, the inverse term “compliance” often is used. Ligament stress-strain: The force-displacement test for ligaments is called a “stress-strain” curve and shows three regions of force-displacement response. Early in loading a small force causes considerable displacement. This is called the “toe region” of the curve. After a certain amount of displacement, the ligament enters the “elastic region” of the curve and becomes markedly more stiff. Finally, if enough force is applied, the ligament begins to fail at its “yield point”. Ligaments “live” in the toe region of the stress-strain curve. This can be seen clinically when, in response to varus-valgus and anteroposterior stress, the tibia moves relative to the femur until it is stopped by tension in the ligament. This is the ligament moving from the toe region into the elastic region. Compliance of the knee: In a number of studies done in the 1970s, the compliance of the knee was found to be least to both varus-valgus and anteroposterior loads in full extension. In flexion, compliance increases particularly to varus-valgus stress. This implies that the ligamentous structures about the knee are most tight in extension and become more lax in flexion. When external load is applied to the knee, either in the form of muscle contraction or bearing weight, the compliance of the knee decreases (i.e., it becomes more stiff and more stable). Loading will decrease the tension in the ligaments, yet the knee is less compliant. The only way this can happen is by the geometry of the surfaces imparting the stability. The conclusion from these studies is that the human knee, when moving in the usual plane of motion, is stabilised by the geometry of the surfaces, or the congruency of the femur and tibia. Ligaments are recruited to limit motion when forces outside the plane of motion (“out-of-plane” loads) are applied to the knee. These loads move the knee ligaments from the toe region into the elastic region of their stress-strain curve.

Two kinds of total knee prosthesis design: Most total knees are designed to have little or no congruence between the femur and tibia, likely because of the worry about “kinematic conflict” that dates to the four-bar-linkage model of knee motion first proposed by Zuppinger in 1907. In these types of total knees, the ligaments are tensioned (i.e., “balanced”) so that they do the job done in the normal knee by congruence. A few total knees are designed for congruence between the femur and tibia, either in just the medial compartment or in both compartments. The answer to the question, “What is needed for total knee stability?” For non-congruent knee prostheses, the ligaments must be balanced or tensioned into the elastic portion of the stress-strain curve so that the knee is stable. The ligaments must remain in the elastic region indefinitely or the knee will be unstable. For congruent knee prostheses, the ligaments can be left in the toe region and rely, similar to the normal knee, on the geometry of the surfaces to provide stability and allow the ligaments to be recruited for out-of-plane loads. The ligaments must not be left too loose, lest the knee be unstable to out-of-plane loads but must not be as tight as is done with ligament tensioning prostheses.

Introduction

Quadriceps performance following total knee arthroplasty (TKA) is a critical factor in patient satisfaction that can be significantly affected by implant design (Greene, 2008). The objective of this study was to compare quadriceps efficiency (QE) following TKA with a medial-pivot system (EVOLUTION®, MicroPort Orthopedics Inc., Arlington, TN, USA) to non-implanted control measurements.

The author's experience is with a specific type of femoral stem and modular neck both fabricated from Ti alloy. Fracture of this Ti-Ti modular neck is associated with heavy weight, heavy activity, long modular necks and corrosion at the junction of the neck to the stem. Instruments have been designed that can remove the distal fragment of the fractured modular neck in most cases.

After the neck is removed, the clinician is faced with the decision to remove the stem or to place another modular neck. At present MicroPort (formerly Wright Medical Technology) the company whose modular neck forms the data for this presentation, suggests that the female part of the taper (that is the part at the top of the stem) should not be reused if a modular neck (fractured or intact) has been removed. Thus the recommendation from the company is that the stem be removed. This recommendation is true across all modular neck – femoral stem combinations particularly for those that have been recalled by the manufacturer.

There is no question that the female taper is changed by having had the modular neck implanted. In most cases of fracture there is severe corrosion at the junction of the neck to stem. Thus, reuse of the taper could have problems and it is understandable that the company does not sanction its reuse. On the other hand, removal of a well-fixed ingrown stem is not without morbidity. In several cases, with patient informed consent, I have left the stem and implanted a new modular neck (either CoCr or Ti alloy) in the damaged taper. We are following these patients closely and it is too soon to make recommendations as to the wisdom of this practice.

My strong recommendation is to remove the stem and place a new one.

Most presentations about total knee arthroplasty begin with a statement that the procedure has been one of the great successes of modern surgery. However, not all patients consider their total knee a success. Success requires that patients experience relief of arthritic pain, return of function, and express satisfaction with the result. Patients need to be aware of the limitations of implants and accept reasonable expectations for the arthroplasty. If they don't, your next revision will likely be on a unsatisfied patient who had unrealistic expectations. The surgeon who operated on the patient for the primary intervention may feel obliged to try to make it better. Don't make that mistake. Avoid your next revision by only intervening when there is a clear indication.

In a recent patient survey, 15–20% of patients (and maybe more) were not completely satisfied with their arthroplasty in spite of having recent implant designs. It is a fact that some patients will not be satisfied with any intervention. Fibromyalgia, depression, high narcotic use for arthritic pain, secondary gain (e.g., Worker's Compensation claims pending) are some of the conditions that predict a difficult post-operative course and an unsatisfied patient who will push for revision. To avoid your next revision, choose patients wisely and make sure they understand that the total joint is a poor substitute for the normal knee.

Design surgeons and engineers have developed techniques for a specific implant system to minimise the problems of malrotation, malalignment, instability, anterior knee pain, stiffness, loosening and polyethylene wear. Surgeons should be careful to use the recommended implantation philosophy and technique to avoid these problems. Choose implant systems with a proven track record. Learn how and why to use the instruments correctly. Study a system well and know the nuances. If you don't know the system well enough, take a course from the designers and ask questions. Avoid your next revision by using a prosthesis system as it was intended.

Prosthetic joint infection remains a major reason for revision. Some patients have a greater chance of developing infection. Attention to detail from pre-operative preparation to rehabilitation will minimise, but cannot eliminate, the occurrence of infection. The recently published International Consensus on Prosthetic Joint Infection contains recommendations that should be followed to minimise the chance of infection. Avoid your next revision by following the recommendations to minimise the chance of infection.

The indication for revision is diagnosis of a problem that can be corrected with surgery. If a patient is satisfied with a result, revision surgery would only rarely be indicated regardless of the radiographic result. (Severe wear would be an exception to this.) Avoid you next revision by recognizing that “the enemy of good is better”.

Most presentations about total knee arthroplasty begin with a statement that the procedure has been one of the great successes of modern surgery. However, not all patients consider their total knee a success. Success requires that patients experience relief of arthritic pain, return of function, and satisfaction with the result. Patients need to be aware of the limitations of implants and accept reasonable expectations for the arthroplasty. If they don't, your next revision will likely be on a dissatisfied patient who had unrealistic expectations. The surgeon who operated on the patient for the primary intervention may feel obliged to try to make it better. Avoid your next revision by only intervening when there is a clear indication.

In a recent patient survey, 15–20% of patients were not completely satisfied with their arthroplasty in spite of having recent implant designs. It is a fact that some patients will not be satisfied with any intervention. Fibromyalgia, depression, high narcotic use for arthritic pain, secondary gain (e.g., Worker's Compensation claims pending) are some of the conditions that predict a difficult post-operative course and an unsatisfied patient who will push for revision. To avoid your next revision, choose patients wisely and make sure they understand that the total joint is a poor substitute for the normal knee.

Design surgeons and engineers have developed techniques for a specific implant system to minimise the problems of malrotation, malalignment, instability, anterior knee pain, stiffness, loosening and polyethylene wear. Surgeons should be careful to use the recommended implantation philosophy and technique to avoid these problems. Choose implant systems with a proven track record. Learn how and why to use the instruments correctly. Study a system well and know the nuances. If you don't know the system well enough, take a course from the designers and ask questions. Using a system as it was intended will help avoid your next revision.

Prosthetic joint infection remains a major reason for revision. Some patients have a greater chance of developing infection. Attention to detail from pre-operative preparation to rehabilitation will minimise, but cannot eliminate, the occurrence of infection. The recently published International Consensus on Prosthetic Joint Infection contains recommendations that should be followed to minimise the chance of infection and thus help avoid your next revision for infection.

The indication for revision is presentation of a problem that can be corrected with surgery. If a patient is satisfied with a result, revision surgery would only rarely be indicated regardless of the radiographic result. (Severe wear would be an exception to this.) Recognising that “the enemy of good is better” will help you avoid your next revision.

Fracture of the Ti-Ti modular neck is associated with heavy weight, heavy activity, long modular necks and corrosion of the junction of the neck to the stem. Instruments have been designed that can remove the distal fragment of the fractured modular neck in most cases. These instruments will be demonstrated in this video.

After the neck is removed, the clinician is faced with the decision to remove the stem or to place another modular neck. At present MicroPort (formerly Wright Medical Technology) the company whose modular neck I used, suggests that the female part of the taper (that is the part at the top of the stem) should not be reused if a modular neck (fractured or intact) has been removed.

There is no question that the female taper is changed by having had the modular neck implanted. In most cases of fracture there is severe corrosion at the junction of the neck to stem. Thus, reuse of the taper could have problems and it is understandable that the company does not sanction its reuse. On the other hand, removal of a well-fixed ingrown stem is not without morbidity. In several cases, with patient informed consent, I have left the stem and implanted a new modular neck (either CoCr and Ti alloy) in the damaged taper. We are following these patients closely and it is too soon to make recommendations as to the wisdom of this practice. My strong recommendation is to remove the stem and place a new one.

Having previously been a proponent of the advantages of the modular neck in total hip arthroplasty, I now take the opposite argument because of corrosion that happens with all taper junctions. The advantage of the modular neck is the “uncoupling” of femoral stem position from the final position of the femoral head. Surgical priorities frequently compete, whether positioning the stem for the best press-fit (for cementless fixation) or the best cement mantle (for cemented fixation), and positioning of the stem for preventing dislocation and improving function. My personal use of the modular neck spanned approximately 4 years from 2003–2008 and encompassed a total of 390 primary and revision cases. Excellent functional results were obtained, but some problems occurred that were associated with the modular neck and with large diameter head metal-metal articulations.

The modular neck was designed and studied at the Rizzoli Institute in Bologna, Italy with the conclusion that the strength of construct (titanium alloy neck in the titanium alloy stem) was sufficient and the potential for fretting at the modular junction was small and acceptable. Pre-market testing of the device met and exceeded all FDA suggested benchmarks.

The first modular neck fracture in my personal series occurred more than 3 years after implantation, in a large man with a long, varus modular neck. Within a year another fracture of a long, varus modular neck occurred in a heavy man. I now know of 6 modular neck fractures among the 390 cases. We have found evidence of corrosion, some very severe, in modular necks that we have revised (both fractured and intact modular necks). This corrosion is caused by Mechanically Assisted Crevice Corrosion associated with fretting at the modular junction which leads to removal of the titanium oxide “passivation” layer that generally forms on a titanium implant. This exposes more of the substrate metal to oxidation and can create pits that, in the notch-sensitive titanium alloy, can lead to the initiation of fracture. The hydrogen that is created from the corrosion reaction and diffused into the metal can cause “embrittlement” which predisposes it to fracture. We also have seen “hydrogen pneumarthrosis” associated with corrosion of the titanium modular neck in which the corrosion concentrated the hydrogen gas in the femoral stem below the modular neck and suddenly was released into the joint with significant pain. The hydrogen gas is irritating to the joint capsule and the patient presents with intense pain and gas in the joint, a clinical picture that can be confused with infection in the joint with a gas-forming organism. We now know that the condition is self-limiting, but suggests that revision of the modular neck construct would be a reasonable course of action.

Recently cobalt chromium modular necks have replaced those made of titanium alloy. Since cobalt-chromium is harder and stiffer, the milieu of the taper junction will be different than that of the titanium-titanium junction, and it has been suggested that this will allow safe and long-term use of the modular neck. The first titanium alloy necks were introduced in the early 1990s and it took until the mid-2000s to recognise problems. Last year the Stryker modular neck used with the Rejuvenate stem was recalled because of significant reaction associated with corrosion at the neck-to-stem junction.

Corrosion is inevitable at modular junctions exposed to cyclic loading, especially in the milieu of body fluids. We now know that ALTR occurs in response to taper junction corrosion as well, and the more modular junctions there are in a total hip construct, the more debris and potential reaction likely. Fixed neck stems provide satisfactory long-term fixation and function for patients, so despite a functional advantage to the modular neck, it is “a bridge too far”.

Anterior knee pain is a frequent complaint of dissatisfied total knee arthroplasty patients. We hypothesize that the need to use the extensor mechanism to stabilise the knee during activity is a cause of anterior knee pain. Studies have shown that TKA patients often walk with a “quadriceps avoidance” gait, which may explain the phenomenon of anterior knee pain.

Most TKA prostheses are designed to allow AP motion. This feature in knee implant design is to prevent the “kinematic conflict” that was predicted with the crossed four-bar-link model of knee motion, which holds that progressive posterior contact of the femur on the tibia (rollback) with flexion was obligatory for knee range of motion. It has been stated that preventing this motion overly “constrained” the knee and could lead to loosening and wear.

Paradoxical motion has been seen with video fluoroscopy in knees after TKA. This motion is an anterior translation of the femur on the tibia early in knee flexion and is called paradoxical because it occurs opposite to the expected rollback. In fact, paradoxical motion is a consequence of the “unconstrained” articulation of the femoral component on the tibial component.

During gait, just after heel strike as the foot is assuming a flat position on the floor, there is a significant vector of force from posterior to anterior. This vector has been calculated as 33% of body weight for walking at normal speed and could lead to a significant displacement of the femur forward on the tibia. It is countered by 1) the slope of the proximal tibia; 2) the articulation of the femur in the concavity of the tibial (with the firmly attached meniscus that deepens the concavity) on the medial side; and 3) the body mass vector combined with that of the contracting quadriceps.

If a total knee prosthesis allows the femur to move forward, the posterior-to-anterior force just after heel strike acts to move the femur forward on the tibia (paradoxical motion). The patient, in an attempt to stabilise the knee, uses increased quadriceps contraction to prevent the forward motion of the femur. The forces required are significant and are not only found in the patella-femoral articulation but all through the retinaculum that covers the anterior part of the femur. As the extensor mechanism tires, patients begin using a quadriceps avoidance gait to adapt to the weakening extensor, and after a period of activity, the stress on the retinaculum leads to pain.

AP stability can be improved through implant design by preventing AP motion through conformity of the femoral and tibial components. We have used a medially conforming ball-in-socket prosthesis as a revision component for patients with anterior knee pain, and have achieved resolution of the pain. Patients demonstrate a “posterior sag” at approximately 20 degrees of flexion (the degree of flexion that has the maximum posterior-to-anterior force during gait). When treated with a brace appropriate for stabilisation of the knee after PCL reconstruction, patients experienced a marked decrease in symptoms and this predicts a good result from revision surgery.

To explain the knee kinematics, the vector of the quadriceps muscle, the primary extensor, is important and the relationship of the quadriceps vector (QV) to other kinematic and anatomic axes will help in understanding the knee.

Knee kinematics is important for understanding knee diseases and is critical for positioning total knee arthroplasty components. The relationship of the quadriceps to knee has not been fully elucidated. Three-dimensional imaging now makes it possible to construct a computer based solid model of the quadriceps and to calculate the vector of the muscle as individual parts and as a whole. Two studies are presented, one American and one Japanese subjects.

Using CT data from subjects who had CT for reasons other than lower extremity pathology (American) or specifically for the study (Japanese), 3-D models of each quadriceps component (vastus medialis, intermedius, lateralis and rectus femoris) were generated. Using principal component analysis for direction and volume for length, a vector for each muscle was constructed and addition of the vectors gave the QV. Three anatomic axes were defined: Anatomic Axis (AA) – long axis of the shaft of the femur; Mechanical Axis (MA) center of the femoral head to the center of the trochlear and the Spherical Axis (SA) – a line from the geometric center of the head of the femur to the geometric center of the medial condyle of the femur at the knee.

Fourteen American cases (mean age 39.1, 9 male 5 female) and 40 Japanese subjects (mean age 29.1, 21 male, 19 female) were evaluated. In all subjects the quadriceps vector at the level of the center of the femoral head was anterolateral to the center of the femoral head. The position of the QV was more lateral in Japanese compared to Americans; and, in Japanese, the vector was more lateral and posterior for women than for men. In both study populations, the QV was most closely aligned with the SA as compared to the AA or the MA.

The vector representing the quadriceps pull, originating at the top of the patella, progresses proximally toward the neck (not the head) of the femur. With the femur in anatomic position in the coronal plane, the vector crosses the femoral neck lateral to the femoral head approximately at the midpoint of the neck. While there were significant differences between the passing point of the vector based on sex and ethnicity, the QV vector most closely parallels the SA (< 1° different) for all subjects in this study. The relationship of the SA to the kinematic flexion axis (KFS) of the knee is being evaluated with the hypothesis that the relationship is 90°. If this is correct, the SA may prove a robust axis to which to align total knee arthroplasty.

We conclude that the QV as calculated progresses from the top of the patella to the mid-femoral neck and the SA is most closely parallel to this vector.