Perspectives on Knee Implant Designs
Lectures, debates, and research on contemporary implant designs for total knee arthroplasty were featured at ICJR’s Pan Pacific Orthopaedic Congress. Below, we highlight 6 of these presentations.
Total Knee Design for Optimal Function
Peter S Walker, PhD; Sally Arno, PhD; and Ilya Borukhov MS
As total knees have evolved since their beginnings in the late 1960s, 2 types of design have emerged as the most frequently used, the posterior cruciate retaining type (CR) and the posterior stabilized type (PS), in which both of the cruciates are resected.
Other designs are available, but are used in much smaller numbers:
- Designs that retain both cruciates
- Medial pivot designs
- Modular components comprising separate bearings for the lateral and medial sides and patella-femoral joints
- Customized total knees
Yet there is evidence that the least-used designs have the capacity to restore normal kinematics more closely than the standard CR and PS designs.
To examine this issue in more detail, we can postulate a design scenario for total knees with the specific objective of restoring the anatomic characteristics of neutral path of motion (the flexion-extension path with only axial compressive loads acting) and the laxity about this neutral path when shear and torque forces are applied. It is assumed that these parameters will contain any type of motion path in any activity, the specific kinematics being determined by body weight, muscle forces, and inertia forces.
To design the “ideal” total knee, a benchmark with sufficient kinematic criteria on normal functions would be required. This benchmark would be used as the target or design criterion for any new design. Either a computer model or a test machine would be needed to test each new design for its compliance with the benchmark.
Based on published data of anatomic knee motion characteristics, it is proposed that a total knee would need these to meet these criteria:
- The total knee would have to have asymmetric femoral and tibial bearing surfaces, whatever the status of the cruciate preservation.
- Throughout flexion, the medial side would need to prevent anterior sliding of the femur but allow some posterior laxity throughout flexion
- The lateral side would need to restrain posterior laxity and allow anterior laxity, yet produce progressive posterior displacement with flexion.
- At all angles, the laxity on the medial side would be less than that on the lateral, while overall laxity would be reduced in extension and high flexion.
A number of design configurations have already been formulated that are closer to the above criteria than standard CR and PS designs. The latter, while usually providing sufficient AP and rotary stability, display little asymmetry in their motion and laxity.
The question for discussion is whether asymmetric designs that would more closely replicate anatomical knee motion characteristics, would have better functional results overall than standard symmetric designs, all else being equal.
Peter S Walker, PhD; Sally Arno, PhD; and Ilya Borukhov MS, are from New York University Hospital for Joint Diseases, New York, New York.
Mechanics of the Knee: In Vivo Kinetics, Kinematics, and Sound
Richard D. Komistek, PhD
Understanding the in vivo mechanics of the knee is very important to knee implant development and the ability to predict longevity of an implant under various loading conditions.
Throughout the past 20 years, our research group has utilized in vivo fluoroscopy as the main input to mathematical models developed using Kane’s Dynamics to predict the in vivo forces and stresses of knee implants during various activities.
More recently, a sound sensor was developed to analyze in vivo vibrations at the bearing surface interface and the sounds associated with each dominant frequency propagated during this interaction. Therefore, the objective of this study was to assess the in vivo mechanics of the knee at both femorotibial and patellofemoral interfaces.
Inverse dynamic mathematical models were developed, using Kane’s Dynamics, for the human lower extremity to assess the bearing surface interaction in the knee joint. Under fluoroscopic surveillance, patients were asked to perform normal gait and deep flexion maneuvers to maximum flexion.
The 3-dimensional position and orientation of the femoral, tibial, and patellofemoral components were defined using in vivo fluoroscopy data, with respect to the Newtonian reference frame. The data were transformed into relative reference frames, defined using temporal functions, and then input into the mathematical model.
Sound sensor data, captured at 10,000 Hz, were also utilized to determine findings of interest that were further assessed using fluoroscopy and the mathematical model.
The maximum knee force for each subject varied considerably both at the femorotibial and patellofemoral interfaces:
During gait, the maximum femorotibial force ranged from 2.1 to 3.2 times body weight (BW), but was significantly greater during a deep knee bend, ranging from 2.5 to 4.5 times BW.
During gait, the patellofemoral force did not increase significantly because knee flexion remains less than 30 degrees during stance-phase of gait, but during a deep knee bend the maximum patellofemoral force ranged from 2.5 to 4.0 times BW.
The quadriceps force remained 1.2 to 2.0 times greater than the patellofemoral ligament during deep flexion maneuvers.
Sounds at the knee resembled clicking, grinding, and cracking.
Determination of in vivo forces at the knee is critical to understandings implant longevity and the viability of implant success. Temporal force profiles derived at the knee joint can be used in knee simulators and finite element analyses to determine in vivo stresses and predict how successful a knee implant may be throughout a patient’s lifetime.
Implanted knee forces are generally much higher than normal knee forces and can vary considerably from patient to patient and implant to implant, leading to the assumption that knee kinematics does play a role in knee implant failure and may be a main reason why one implant last longer than another.
Richard D. Komistek, PhD, is from the Center for Musculoskeletal Research, University of Tennessee, Knoxville, Tennessee.
GREAT DEBATE: Does the ACL Belong with a Total Knee Arthroplasty?
Yes: Harold E. Cates, MD
An intact anterior cruciate ligament (ACL) is an important part of the 4-part link system in normal knees. Early attempts at saving both cruciate ligaments in total knee arthroplasty (TKA) were complicated by poor design, inferior polyethylene quality, and rudimentary surgical techniques. Successful bicruciate sparing total knee arthroplasties have been previously demonstrated to have knee kinematics most similar to the normal knee.
Up to 20% of patients are dissatisfied with their TKA. All current TKA designs – CR, PS and MB – demonstrate inferior knee kinematics compared with the normal knee.
New developments – such as contemporary design and material changes, better locking mechanisms, improved polyethylene properties, and enhanced surgical instrumentation – make retention of the ACL a viable option for TKA.
Harold E. Cates, MD, is from Tennessee Orthopaedic Clinics, Knoxville, Tennessee.
No: W. Norman Scott, MD, FACS
Cruciate retaining (CR) prostheses were the rule in the early history of total knee arthroplasty (TKA).
The theory behind the design concept of retaining the posterior cruciate ligament (PCL) – and the anterior cruciate ligament (ACL) as described in the 1970s – was that, by retaining these cruciate ligaments, one would achieve normal stability, better stair climbing capability, normal proprioception, and a reproducible surgical technique.
These theories have not been proven accurate. Compared with posterior stabilized (PS) designs, the CR (PCL retaining) designs have a higher dislocation rate and show no difference in stair climbing capabilities and no significant improvement in proprioception. In addition, the surgical technique to retain an “arthritic” ligament is more difficult.
Reality reflects these conclusions, as evidenced by the dramatic shift in CR vs. PS designs, with CR designs accounting for 38% of implant utilization compared with 53% for PS designs. 
There is presently increasing interest in revisiting design techniques to utilize the normal ACL and PCL in a bicruciate retaining knee. The theoretical reasons reflect the CR ideology of the 1970s, 1980s, 1990s, and the present.
Unfortunately, we know that in the arthritic knee, the ACL (similar to the PCL) is not normal histologically or even via MRI imaging.
The surgical technique is extremely demanding and somewhat unreproducible. Thus, with the overwhelming success of PS TKA, it does not make sense to try and preserve an abnormal ligament rather than utilizing an effective knee design.
- Nguyen LC, Lehil MS, Bozic KJ. Trends in total knee arthroplasty implant utilization. J Arthroplasty. 2015 May;30(5):739-42. doi: 10.1016/j.arth.2014.12.009. Epub 2014 Dec 13.
W. Norman Scott, MD, FACS, is from the Insall Scott Kelly Institute, New York, New York.
Guided Motion in Total Knee Arthroplasty
Michael D. Ries, MD
Total knee arthroplasty (TKA) is an effective treatment for arthritic knee pain, but after TKA, patients may not return to their desired recreational or functional activity level.
The differences in functional outcome between TKA patients and normal subjects may be related to differences in kinematics between the normal and replaced knee. In the normal knee:
The lateral condyle of the femur rolls posteriorly during knee flexion, which is associated with femoral external rotation.
Posterior movement of the femur increases the moment arm of the quadriceps
Femoral external rotation (or tibial internal rotation) causes relative medialization of the tibial tubercle, which facilitates patellar tracking in flexion.
However, after TKA, the knee typically behaves as an anterior cruciate ligament (ACL) deficient knee. Posterior cruciate retaining TKAs are associated with “paradoxical motion” in which the tibia is subluxed anteriorly in extension and the femur translates anteriorly during knee flexion.
Unicompartmental and patellofemoral arthroplasty, which retain both cruciate ligaments, provide relatively normal knee kinematics and favorable knee function compared with conventional TKA that sacrifice the ACL. Guided motion TKA designs rely on the geometry of the tibial and femoral articular surfaces and cam post mechanism to provide more normal knee kinematics and tibiofemoral AP stability.
Clinical results with a first-generation bicruciate substituting TKA have generally been favorable.  Fluoroscopic analyses of in vivo kinematics demonstrate AP displacement and rotation similar to the normal knee.  However symptomatic iliotibial (IT) band tendonitis and instability have also been observed, which likely is related to tension in the periarticular soft tissues during functional activities. 
A second-generation bicruciate substituting TKA provides femoral rollback and rotation similar to the normal knee, with relatively normal ligament tension throughout the arc of knee motion. . Early clinical results with this design have been quite favorable, with a relative absence of IT band tendonitis or instability, suggesting that effective guided motion after TKA requires both normal kinematics and soft tissue tension.
- Mugnai R, Digennaro V, Ensini A, leardini A, Catani F. Can TKA design affect the clinical outcome? Comparison between two guided-motion systems. Knee Surg Sports Traumatol Arthroscopy 2013.
- Catani F, Belvedere B, Ensini A, Bianchi L, Giannini S. Effect of guided knee motion TKR design on knee kinematics. ORS, KS Coventry Award, 2008.
- Luyckx L, Luyckx T, Bellemans J, Victor J. Iliotibial band traction syndrome in guided motion TKA. A new clinical entity after TKA. Acta Orthop Belg, 76: 507-512 , 2010.
- Ries MD, Hughes D, Laster S, Lenz N. Effect of component design and kinematics on mid-flexion ligament tension in primary TKA. ICJR Pan Pacific Orthopaedic Congress, 2014.
Michael D. Ries, MD, is from the Tahoe Fracture & Orthopaedic Clinic, Carson City, Nevada.
Early Experience with Bicruciate Retaining TKA
Craig J. Della Valle, MD; Thomas P. Andriacchi, PhD; Keith R. Berend, MD; Jeffrey H. DeClaire, MD; Adolph V. Lombardi Jr., MD, FACS; Christopher L. Peters, MD
Bicruciate retaining total knee arthroplasty (BiCrTKA) has recently been re-introduced into the armamentarium of the orthopaedic surgeon for treatment of end-stage knee arthritis for patients with an intact anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL).
As with the introduction of any new technology into orthopaedic surgery, there is a concern that the technique may be associated with a learning curve and adverse outcomes. The purpose of this report is to describe our initial experience with BiCrTKA.
Following clearance by the US Food and Drug Administration, 6 surgeons implanted a cemented BiCrTKA with patella resurfacing in 383 patients (67% female, mean age 65 years +/- 8.6) between May 2013 and April 2014, and followed them for a minimum of 90 days.
After the first 119 cases, the surgeons discussed the adverse outcomes and the surgical technique was re-assessed. The rate of complications prior to and following the change in technique was compared using a chi square test with P-values < 0.05 considered significant.
The most commonly identified complication was intraoperative fracture of the bone island. There were 11 island fractures among the first 119 cases, compared with 5 in the subsequent 258 cases (9.2% vs. 1.9%; p = 0.001).
There were 4 manipulations performed for range of motion <90° in the first group, compared with none in the second group (3.4% vs. 0%; p =0.003). Two reoperations were performed overall (0.5%; 1 for instability and 1 for tibial loosening), both in the first group.
Mean operative time decreased from 82.7 minutes in the first group to 77.5 minutes in the second group (P = .031).
Our initial experience with BiCrTKA was associated with a low risk of overall complications that was decreased significantly with a greater understanding of and changes to the surgical technique.
The most common complication observed was intraoperative bone island fracture that was easily converted to a standard TKA.
Craig J. Della Valle, MD, is from Rush University Medical Center, Chicago, Illinois. Thomas P. Andriacchi, PhD, is from Stanford University, Stanford, California. Keith R. Berend, MD, and Adolph V. Lombardi Jr., MD, FACS, are from Joint Replacement Surgeons, New Albany, Ohio. Jeffrey H. DeClaire, MD, is from the DeClaire LaMacchia Orthopaedic Institute, Rochester Hills, Michigan. Christopher L. Peters, MD, is from the University of Utah, Salt Lake City, Utah.