Zurich Cementless THR Principles
MAIN DESIGN OBJECTIVES
(1) Provide an immediate and indefinitely stable bone anchorage of the femoral and acetabular components.
(2) Minimize wear of the artificial joint.
The Zurich Cementless THR stem achieves permanent anchorage on the femoral side through bony ingrowth from the medial cortex without coupling to the lateral cortex. Stability required for ingrowth is guaranteed by locking screw fixation of the stem to the cortex from the inner side of the bone**. This results in near physiological loading of the proximal femur, i.e. absence of stress shielding.
On the acetabulum side, the outer shell of the cup is manufactured from perforated, highly compliant, titanium, with an inner non-perforated shell and an ultra-high molecular weight polyethylene (UHMWPE) inner lining to receive the head of the stem. The double-shelled design** provides for rapid and consistent integration of the acetabular bone into the outer shell of the cup.
Minimizing wear reduces the risk of bone lysis mediated aseptic loosening. Instead of a conventional spherical shape, an artificial Fossa™** provides an articulating surface that minimizes contact with the femoral head and improves hydrodynamic lubrication within the cup. This modified geometry of the contact area between the head of the prosthesis and the polyethylene liner reduces the contact stresses several fold. A new amorphous diamond-like coating (ADLC) provides increased hardness and lubricity to the head/cup interface, also reducing wear.
INNOVATION AND DEVELOPMENT
Achieving the previously stated design objectives required solving two, partially coupled problems:
(1) Eliminating load-induced movement at the bone-implant interfaces, and
(2) Minimizing stress shielding of the bone, particularly the shielding caused by conventional, stemmed femoral components.
Bone cement in a Charnley-type cemented THR accomplishes both: stability of the interface by an in situ polymerized interlock; and well defined load distribution through a compliant cement mantle. This is fine for the near term, but less satisfactory on a long term basis. Aseptic loosening is the most common reason for long-term failure of THR surgery. This is well documented for human hip surgery and has been recently demonstrated to be even more so for canine THR. The high aseptic loosening rates of cemented hips has been the main driving force behind development of cementless THR. However, clinical performance with sufficient follow-up of all the different types of cementless THRs shows this technique to be inferior to that of a well-designed and well cemented THR. In most cases, cementless prostheses have replaced the soft cement mantle by adding more stiff metal to an already stiff core element. This exacerbates both problems. A higher mismatch in stiffness leads to more pronounced stress shielding and higher shear loads at interfaces, increasing the risk of micromotion. The femoral and acetabular components of KYON THR reflect different, novel approaches to the design objectives.
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ANCHORAGE OF THE FEMORAL COMPONENT:
During normal cycling, the femur is subjected to compression loading on the medial side and tension forces on the lateral side. The femur is naturally more compliant than a solid, canal-filling metal prosthesis. Physiological loading generates high interface shear stresses, which can be resisted initially by friction alone. It has been shown by theoretical analysis that canal-filling, press-fitted, metal stems cannot be stable at all contact areas with the femur under physiological-level loading. Should any motion occur before the interface is secured by bone adaptation to the implant (by ongrowth and/or ingrowth), true, solid anchorage of the prosthesis will fail.
Preparation of the medullary canal for implantation kills about two thirds of the cortex with the endosteal blood supply to the bone inevitably destroyed. About 10 to 12 weeks after surgery, remodeling of this dead bone will lead to its peak porosity. During this period, and then longer still to allow for the bone to refill and gain some strength, the hip should be protected from (over) loading. This is at best difficult, and in most cases impossible. As a consequence, most, if not all, of press-fit implants get loose at some stage and are then subjected to a chancy process of bone remodeling which may eventually form a stable interface at some areas, with soft connective tissue covering most of the implant.
In departure from the press-fit/bony ingrowth concept, the KYON Zurich Cementless THR deploys screw-based primary fixation of the femoral component. The Zurich Cementless THR uses locking screws. Conventional screws, used for hip prosthesis fixation in the 1950s, caused bone remodeling around the screws, resulting in loss of stability and implant failure. But with screws safely locked in the stem, the mechanics is similar to the extensively researched PcFix plating system of AO/ASIF. Remodeling can proceed without a major risk of loosening, without bone resorption occurring around all of the screws at the same time. The Zurich Cementless THR is fixed by mono-cortical screws to the medial cortex only. Screws are passed through the access holes in the lateral cortex, all drilling and fixation is performed with the aid of a drill guide attached to the stem. Since the stem does not touch the lateral cortex, it can move freely, does not transfer any load to the cortex, and does not cause bone resorption. Avoiding coupling between the medial and lateral cortices is the most important, distinguishing characteristic of the Zurich Cementless THR.
If any bone is to eventually bridge the gap and grow to direct apposition with the implant, it may do so under ideal conditions of stability. To facilitate the process of integration the stem is plasma coated with pure titanium.
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ANCHORAGE OF THE ACETABULAR COMPONENT:
In conventional cementless acetabular components, the subchondral shell is either completely removed in designs aiming for bone ingrowth, or partially retained in various threaded-type designs. The metal backing is usually a very stiff structure leading to a huge mismatch in compliance and seriously reducing the chances of a complete, long-lasting bony integration.
In most cases, the metal backing of conventional cups engage bone with a textured surface, sometimes with interconnected pores running some depth into the material, but ending in closed, dead-end holes. Our preoccupation with the role of convective transports in bone growth and remodeling has led us to propose the concept of a hydraulically open implant – the acetabular component of the Zurich Cementless THR being the first embodiment of this concept.
The polyethylene (UHMWPE) insert is suspended within a double layer of titanium. The first layer is a thin smooth surface to prevent the proginator cells from interacting with the polyethylene material. The outer layer is a densely perforated titanium shell leaving about 1 millimeter of free space between the inner wall of the metal shell and the outer wall of the smooth titanium insert, i.e. bone is free to grow past the shell into this space. Ingrowth is accelerated by convective fluid currents, set in motion by cyclic pressure gradients caused by the physiological loading of the bone. This is perhaps the main functional distinction over the perforated, cylindrical implants developed by Franz Sutter (who has also supported our early efforts) of the Straumann Institute, Waldenburg, Switzerland, mostly for dental, but also for orthopedic applications. Fluid convection is presumed to increase mass transport of important bone ingrowth promoting factors emanating from the extant cancellous bone surrounding the implant.
The surface of the outer titanium shell is plasma, titanium coated for an additional microinterlock with the bone. For improved press-fit the shell incorporates small protrusions running circumferentionally just below the equator. The pole of the shell is slightly flattened to avoid the cups bottoming out at the pole without a full engagement at the equator.
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The design objective for articulation is to maximize the range of impingement-free motion by minimizing the neck diameter, with maximum head coverage. This objective has been met in steps, with the final optimization of the neck shape to reduce stress concentration and the UHMWPE insert providing approximately 200° of cover. The combination allows for over 120°, now 135° with BIG HEADs, of impingement-free angulation. The average luxation rate is about 5% with several surgeons reporting it as low as 1%. We have found a new challenge in meeting stability criteria in very young – 6-10 months – dysplastic dogs, which typically show a great range of motion, and therefore an increased risk of luxation. A series of intra-operative stability tests, and appropriate corrective measures, if any lack of stability is detected, provide a strong guarantee against postoperative luxation.
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 – Annual Report 2002, The Swedish National Hip Arthroplasty Register, Dept. of Orthopaedics, Sahlgrenska University Hospital, April 2003, www.jru.orthop.gu.se
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