Martin Ma & Hjalmar Pompe van Meerdervoort
Vice President, Global Hips and U.S. Marketing & Vice President, Global Knees and U.S. Marketing, Zimmer, Inc.
Q: What does personalized medicine mean for the orthopaedic industry?
A: HP: The drive to develop tailored implants and instrumentation has come as much from the surgeons as it has from the patients. Delivering personalized orthopaedic care means giving surgeons the freedom to choose the interventions, technologies and product solutions that best match each patient’s unique anatomy, pathology and medical history. It also means delivering on the patient expectation of a smooth transition back to their lifestyle.
At Zimmer we believe that this personalized approach to orthopaedics begins and ends with implant designs that draw directly from the body’s natural biomechanics whenever possible. For instance, the NexGen® Knee System helps restore natural knee kinematics while providing high flexion for patients whose lifestyles require higher flexion capabilities.
MM: As the market leader in knee and hip replacements, we see it as our responsibility to develop the flagship products in this growing market of personalized care. The Zimmer® M/L Taper hip implant with Kinectiv® Technology features a broad range of modular neck options to treat patients with a personalized hip fit while minimizing soft tissue trauma and enabling the surgeon to adjust leg length, offset and version independently.
Q: How is the Zimmer M/L Taper with Kinectiv Technology different from competing stem systems?
As you know, total hip replacement involves distinct surgical challenges including restoring leg length, joint stability and range of motion. The Zimmer M/L Taper with Kinectiv Technology introduces a system of modular neck components in a familiar wedged-shaped, tapered stem designed to help surgeons restore the hip joint center by addressing the foregoing issues independently. With traditional hip systems, leg length and offset are coupled, and surgeons are left to accept the coincidental change in leg length or offset when making changes intraoperatively. As illustrated in Exhibit 1, Zimmer’s system allows surgeons to independently adjust leg length and offset intraoperatively in order to optimize each factor without affecting the others. The surgeon can optimally fill the metaphyseal region with the desired stem size and identify a head center that will appropriately restore leg length and offset. In short, Kinectiv Technology allows the surgeon to fit the implant to the patient instead of the patient to the implant.
The ability to match a wide range of patient anatomies and intraoperative flexibility is what distinguishes the M/L Taper with Kinectiv Technology from other implants.
Q: How does the M/L Taper with Kinectiv Technology meet Zimmer’s commitment to personalized orthopaedics?
MM: This system was designed to help surgeons address a wide range of bone morphologies. For example, Exhibit 2 illustrates that women tend to have lower head centers, less offset and greater anteversion, whereas men tend to have greater offset and less anteversion.1-7 The short leg length neck options available in Zimmer’s system have been designed for surgeons to address the challenging varus femoral anatomy, commonly seen in female patients, without having to perform low, bone-sacrificing osteotomies to restore leg length. These gender-specific tendencies are profound, but we also recognize that there is a wide anatomical variation within male and female populations. These are addressed by offering head centers that better match the full range of anatomies for both men and women. Our system offers 60 head center solutions with five offset options for a given leg length, five leg length options for a given femoral offset in 4mm increments, and anteverted and retroverted options for each leg length/offset combination.
However, it’s not simply about treating the predictably challenging cases. Personalized care is also about addressing the unpredictable anatomical condition or implant position. The innovative system of modular necks allows the surgeon to efficiently respond intraoperatively to excessive femoral version, muscle laxity or contracture, discrepancies with x-ray templating and unanticipated cup version, depth or height. The intraoperative flexibility of independent leg length, offset and version aids the surgeon to equalize leg length, reduce the incidence of impingement and optimize range of motion unlike ever before.
Q: What effect does the Kinectiv modular head technology have on range of motion?
A: MM: The keystone to Kinectiv Technology is the use of +0 femoral heads exclusively. Range of motion and joint stability are primary issues in total hip replacement. With the sole use of the +0 head, every neck geometry has been optimized to this head length to benefit range of motion. This also eliminates
the use of skirted femoral heads that are necessary for the longer offset option in competing systems. The ante/retroverted necks allow the surgeon to intraoperatively fine-tune the range of motion depending on the patient anatomy and implant position. (See Exhibit 3.) The versatility of a modular neck system allows the surgeon to restore leg length, offset and version to create a well balanced soft tissue environment. By combining the most accurate head position with the best neck option for the patient, surgeons can optimize range of motion and minimize the likelihood of instability and dislocation.
Q: Is there anything further about the Zimmer M/L Taper with Kinectiv Technology?
A: MM: I think it’s important to stress that Zimmer offers this increased flexibility to surgeons without introducing increased complexity. We’ve kept it simple. The Kinectiv Technology implants and instrumentation have been designed with significant consultation from surgeons, O.R. personnel and sales representatives. Our goal was a streamlined, simple option for both surgeon and patient. Our sole use of the +0 heads gives the system that simplicity and eliminates the need for multiple head lengths, complicated charts and head size limitations. We have been quite pleased by the response from surgeons and O.R. teams who have found that the intuitive and readily apparent 4mm leg length and offset adjustments help improve operative flow.
Q: Zimmer isn’t the first to develop a mobile bearing knee implant. Why a new generation of mobile bearing knees?
A: HP: There have been several generations of mobile bearing implants, and as an industry we’ve learned that, for certain patient
demographics, a mobile bearing option can provide a more natural range of motion. As the market leader in total knee arthroplasty, we feel it’s imperative to offer a solution for this market segment that builds on the clinical legacy of our NexGen Knee System while offering unique and value added technologies. The next step is improving upon patient satisfaction and the implant function and performance expectations. To this end, the NexGen LPS-Flex Mobile Bearing Knee System (See Exhibit 4.) was designed to accomplish three main goals: (1) to restore natural kinematics, (2) to accommodate safe flexion to 155°, and (3) to enable surgeons to utilize Minimally Invasive Solutions™ procedures.
Q: Tell us more about the benefits of the NexGen LPS-Flex Mobile Bearing Knee.
HP: The NexGen LPS-Flex Mobile Bearing Knee System was the result of our analysis of the effects and component interactions that result when knee prostheses undergo deep flexion, beyond 120°. Zimmer’s design allows for safe high flexion as the posterior condyles roll back up to 155°. This higher flexion meets the needs of patients with active daily living, leisure or work activities, as well as patients whose culture or social customs dictate frequent kneeling or sitting cross-legged.
Antecedent mobile bearing designs have failed to address serious functional limitations arising from the kinematics of knee rotation.
Zimmer has designed the NexGen LPS-Flex Mobile Bearing Knee with an anterior pivot (See Exhibit 5.), which is where body mechanics tell us it should be – near the anterior cruciate ligament site. Anterior pivoting replicates the anatomic center of rotation and reduces patellofemoral forces.8-11 Patellofemoral forces also increase with greater medial/lateral displacement, which research has implicated in post-surgical anterior knee pain. Reduction in patellofemoral forces could reduce patellar complications such as patellar dislocation, component wear, damage and loosening.8,9 An anterior stop on the tibial baseplate effectively prevents the “spinout” seen in competing implants and noted in the literature.12,13 In regards to design features which facilitate minimally invasive surgical approaches, the height of the anterior trunnion is only 10mm, which in combination with our “mini keel” tibial baseplate (See Exhibit 6.) reduces the need for tibial subluxation to insert the components. Another key feature is Zimmer’s enhanced PS cam/spine mechanism which drives down the spine of the articular surface, increasing resistance to subluxation.
Q: Some companies claim that bone conservation is a key factor in high-flex knee implants. These companies criticise femoral designs such as the LPS-Flex because an additional 2mm of bone must be removed from the posterior condyles compared to the standard femoral designs.
HP: While bone conservation is always important, Zimmer research has shown the removal of an additional 2mm of bone is required to optimize the tibiofemoral contact area (See Exhibit 7.) throughout the entire range of motion. An implant system that provides a greater tibiofemoral contact area during high flexion can help minimize the possibility of edge loading, and thereby reduce the likelihood of the metal condyle “digging” into the articular surface when the knee is flexed beyond 125°.14 On the NexGen LPS-Flex implants, this was accomplished by extending the radius of the posterior femoral condyles so the contact area remains high in deep flexion.
To extend the radius, the posterior condyles are slightly thicker, which also increases the posterior condylar offset, and thus reduces the chance of impingement of the femoral shaft on the tibial articular surface.15
Q: Let’s talk about the procedure. What kind of instrumentation and procedure options are there for surgeons in implanting the NexGen Mobile Bearing Knee?
A: HP: The NexGen Mobile Bearing Knee flexibly accommodates different surgical styles, and can be accomplished through a minimally invasive procedure which generally results in smaller incisions, less blood loss and less pain. The Zimmer Multi-Reference® 4-in-1 Instrumentation allows for a measured resection technique. Alternatively, the Zimmer Minimally Invasive Solutions Intramedullary Instrumentation accommodates a tibia-first, flexion balancing approach. Regardless of the surgical technique, this precise instrumentation allows surgeons to adjust and control alignment prior to performing any bone cuts.
Q: What’s next for Zimmer?
A: HP: We are currently developing new products and technologies across all of our reconstructive lines, with some exciting offerings coming out of the extremities category, as well as knees and hips. We are also continuing to build our global spine, trauma and dental businesses. As we continue to grow across all divisions, we are also investing in orthobiologic technologies and advanced materials, allowing us to address more of the continuum of care for joint disease.
MM: In parallel with our innovations in orthopaedic solutions, we will continue to expand our offerings at the Zimmer Institute, which is setting the standard in the industry for personalized and hands-on surgical training.
- Noble PC, Box GG, Kamaric E, Fink MJ, Alexander JW, Tullos HS. The effect of aging on the shape of the proximal femur. Clin Orthop. 1995;316:31-44.
- Maruyama M, Feinberg JR, Capello WN, D’Antonio JA. Morphologic features of the acetabulum and femur: anteversion angle and implant positioning. Clin Orthop. 2001;393:52-65.
- Data from Mohamed Mahfouz, Ph.D., University of Tennessee Center for Musculoskeletal Research. Femoral Bone Atlas.
- Data on file at Zimmer.
- Sugano N, Noble PC, Kamaric E. Predicting the position of the femoral head center. J Arthroplasty. 1999;14:102-107.
- Dorr LD, Long WT, Inaba Y, Sirianni L, Boutary M. MIS total hip replacement with a single posterior approach. Seminars in Arthroplasty. 2005;16:179-185.
- Malik A, Maheshwari A, Dorr LD. Impingement with total hip replacement. J Bone Joint Surg. 2007;89:1832-1842.
- Smith AJ, et al. Pre-surgery knee joint loading patterns during walking predict the presence and severity of anterior knee pain after total knee arthroplasty. J Orthop Res. Number 22, pp 26-266. 2004
- Browne C, et al. Patellofemoral forces after total knee arthroplasty: effect of extensor moment arm. Knee. Number 12, pp 81-88. 2005.
- Scuderi GR et al. In vivo kinematic evaluation and design considerations related to high flexion in total knee arthroplasty. J Biomech. Number 38, pp 277-284. 2005
- Hollister AM, et al. The Axes of Rotation of the Knee. Clin Orthop Relat Res. Number 290, pp 259-268. 1993
- Stiehl JB. Clinical results and complications in mobile bearing total knee arthroplasty. Instr Course Lect 2005;55:233.
- Thompson N.W., Wilson DS, Cran GW, et al. Dislocation of the rotating platform after low contact stress total knee arthroplasty. Clin Orthop. 2004;425:207.
- Data on file at Zimmer.
- Bellemans J, Banks S, Victor J, Vandenneucker H, Moemans A. Fluoroscopic analysis of the kinematics of deep flexion in total knee arthroplasty: influence of posterior condylar offset. J Bone Joint Surg (Br). 2002; 84-B(1):50-53.
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