Cementless Fixation of Total Hip Replacements - Essay Example

Cementless Total Hip Joint Arthroplasty Table of Contents Page Introduction 3 Fixation 3 Physiology ofthe cementless system 5 Hydroxyapatite coating 5 Porous coating 6 Location and extent of bone growth 8 Advantages of uncemented fixation 9 Disadvantages of uncemented fixation 10 References 10 Introduction The hip joint contains the articulating surfaces of the spheroid femoral head and acetabulum cavity. The spherical head has approximately 40% coverage in the acetabulum in any position. It is one of the body's largest weight-bearing joints. The articulation in this joint allows a person to walk, squat, and turn without pain (Canale, 1998). Defects of the hip's cartilage can be caused by osteoarthritis, rheumatoid arthritis, avascular necrosis, and trauma (Fitzpatrick etal, 1998, 272) Fixation Probably no issue is debated more widely in joint replacement surgery than whether an implant should be fixed to the host bone with or without cement. Literature supporting or refuting both philosophies is available. Cemented fixation - Cemented stems have a variety of smooth, textured, and coated surfaces that bond to a layer of cement. These stems occupy 80% of the medullary canal to allow for a mantle (ie, cement-occupied space). A centralizer is added to many cemented stems to keep the stem in the center of the canal, which provides a uniform space for the cement around the implant. (Canale, 1998, 314). Differing philosophies guide surgeons in selecting cemented femoral implants for patients. There was a trend in the 1980s to use more cementless implants; in the 1990s, cemented implants regained popularity. Currently, surgeons are favoring cementless techniques again as a result of proven extended service life in long-term outcome studies on porous implants. Cement is indicated when a patient's bone quality cannot be stabilized satisfactorily with a cement-less implant (Wheeless', 2003). The basic principle of uncemented fixation is that the initial stability of an implant is achieved by mechanical interlock and initial apposition of implant surfaces to host bone is converted to long-term stability by the ingrowth/ongrowth of a stable biological interface (Bloebaum etal, 1997) Cementless or pressfit fixation - with use of cemented impant systems, problems related to cement fixation, including loosening, bone loss, and signs of fragmented cement, were identified. Research efforts led to fixation without cement by using femoral stems tightly fit into the canal. This technique often is referred to as pressfit. Pressfit femoral stems have a porous surface that allows bone ingrowth into the stem, referred to as biological fixation (Hoffmann, 2000). One of the coating systems for implants is calcium phosphate-based material called calcium hydroxyapatite. Hydroxyapatite is the inorganic phase of bone, is inherently compatible with the body, and has been shown to promote bone growth and enhance implant fixation (Geesink etal, 1987). It is used as an additional way to bond bone biologically to a stem and cup. These stems provide immediate implant stability and fit tightly to the endosteal cavity of the proximal femur. There are four basic shapes for press-fit femoral stems-anatomical (ie, bowed), modular, straight, and tapered (Canale, 1998, 315, Hoffmann etal, 2000). The anatomical stem has experienced increased popularity and market growth as a result of excellent long-term results documented in the literature (Hoffmann etal, 2000). The porous coated stem previously was indicated for use in active, young patients and for revision of cemented hips, but now surgeons are using pressfit stems in patients of all ages who have good bone stock (S Rapp, 2003, 14). Physiology of the cementless system Micromotion of 20 m or less at the bone-implant interface will create an environment in which stable bone ingrowth can occur, that motions of 40 m lead to less stable interfaces, and that motions of 150 mm will prevent this ingrowth (Jasty et al. 1997). Attempts to improve bone ingrowth into metal implants have centered on porous coating and/or coating with calcium phosphate ceramics, of which the most important is hydroxyapatite. Hydroxyapatite coating Hydroxyapatite is unsuitable as a load-bearing material but when used as a surface coating provides a biocompatible interface which encourages bone ingrowth. Hydroxyapatite is osteoconductive but not osteoinductive. The optimum thickness of hydroxyapatite for coating a prosthetic component is accepted as being around 50 mm (Geesink et al. 1987). Chromium-cobalt alloy or titanium is considered most suitable for application of a hydroxyapatite coating. Surface modifications such as microstructuring, macrostructuring, or porous coating can be used to improve the bonding of the hydroxyapatite to the implant. Failures of hydroxyapatite interfaces due to debonding, fracture, and rapid degradation of the hydroxyapatite have been observed (Geesink etal, 1987). The risk of fracture of the hydroxyapatite can be diminished by avoiding layers which are too thick; however, rapid degradation has also been associated with hydroxyapatite layers less than 30 mm thick (Hoffmann, 2000) . Porous coating Like hydroxyapatite coating, porous coating is most commonly performed on chromium-cobalt or titanium component (Pilliar, 1983). The aim is that the final construct retains enough strength to tolerate applied loads whilst the porous coating enhances bone ingrowth and implant-bone interface strength. The porous coating usually consists of spherical chromium-cobalt or titanium beads or titanium mesh. Sintering and diffusion bonding are commonly used to apply the porous coating. A pore size of 50 m is accepted as being the minimum for bony ingrowth, with the ideal pore size for ingrowth being in the range from 50 to 400 m (Pilliar 1983). Most retrieval studies show that bony ingrowth only occurs into around 10 to 20 per cent of well-fixed porous-coated implants (Bloebaum et al. 1997). Degradation of the metal component due to the porous coating process, increased production of metal ions due to the enlarged surface area of the component, and failure of the porous-coated interface are the principle concerns expressed with regard to porous-coated implants. The cementless fixation procedure may involve procedures involving both the acetabulum and as well as the femoral head, and involve different issues .Initial fixation of acetabular components is gained in one of two fundamental ways, press-fitting or screwing in a threaded component. Threaded components have been associated with high rates of failure (Morscher 1992). Currently, the attainment of the initial press-fit or the cementless fixation method of an acetabular component is most commonly, and successfully, achieved by under-reaming the acetabulum relative to the component and using a hemispherical shell. The acetabulum should be under-reamed by 1 to 3 mm depending on the quality of the patient's bone and which implant is being utilized ( Canale, 1998) . The principle evidence in favor of screw fixation is reports of enhanced bony ingrowth in the region of acetabular shells adjacent to screws. The principle arguments against the use of screws are that they may lead to further instability of the component rather than stabilizing it (Murray, etal, 1995). In an uncemented system, there are concerns of loss of acetabular bone due to wide reaming required for the implant. Thus if the patient is an elderly one, cementless fixation needs to be done with caution because of reduced bone stock in these patients. Initial mechanical stability of a femoral component is achieved in essentially one of two ways-diaphyseal or metaphyseal press-fit fixation (Engh etal, 1987). The majority of the stems which achieve diaphyseal (distal) fixation are extensively porous-coated straight stems, whilst the majority of stems which achieve metaphyseal fixation are fully proximally porous coated and designed to achieve proximal fit and fill Diaphyseal fixation - aims to achieve stability by press-fitting the femoral component into the diaphysis of the femur. The quality of the initial fixation can be enhanced by using flutes or changing the surface finish. The greatest problems associated with diaphyseal fixation are proximal stress shielding, thigh pain, and difficulties associated with removal of well-fixed components. Diaphyseal fixation leaves the proximal femur relatively unloaded (Sychterz etal, 2002). As bone which is not loaded becomes osteoporotic, it is common to see proximal metaphyseal loss of bone around distally fixed uncemented components. A recent report (Bugbee et al. 1997) suggests that the stress shielding seen proximally around one such component is not progressive and is not associated with increased failure Metaphyseal fixation - aims to fashion the proximal femur to achieve maximal surface contact with the prosthesis. The anatomy of the proximal femur is variable, which means that it is impossible to achieve a perfect match between implant and femur. Circumferential surface coating has been associated with lower rates of distal osteolysis than has partial coating. This is probably because the fully coated implants block pathways of migration of particles generated at the articulating surface (Wheeless' 2007) Location and extent of bone growth In a study by Engh etal (1995), Scanning electron microscopy of femoral components howed that a mean 35% 5% of the porous-coated surface had bone ingrowth. For extensively-coated stems, an average of 37% of the porous-coated surface was bone ingrown; for proximally-coated stems, the average was 30%. In proximal sections, the pattern of ingrowth was less predictable but was observed most frequently on the rounded medial and lateral corners of the implant. In locations where the implant was distant from the outer cortex (because of the differences in its design and the anatomic shape of the proximal femur), areas of ingrowth were connected to the proximal femoral cortex by hypertrophied cancellous trabeculae. With the concept of osseointegration confirmed histologically, the mechanical stability of porous-coated stems was tested to assess the durability of this new type of fixation. For bone-ingrown components, autopsy studies revealed very stable implants with little micromotion. By mechanically testing 13 bone-ingrown implants (Whiteside etal , 93) reported a maximum of only 4 m of micromotion when mechanically testing a bone ingrown proximally coated implant. Overall, the findings indicated excellent fixation of implant to bone with little or no motion at the bone-implant interface. Thus cementless fixation has been shown to be stable in long term studies. Osteolysis Osteolysis has been an area of considerable interest in hip arthroplasty in recent years (Bugbee, 1997), have found that significant proximal osteolysis was uncommon and distal osteolysis did not occur suggests that bony or stable fibrous integration with extensively coated femoral components can protect against distal osteolysis. In no case did osteolysis affect femoral component stability adversely Advantages of uncemented fixation 1 associated with better short and medium term stability (Maloney, 1996) 2 no problem of cement particles getting loose and implant instability due to this (Rapp, 2003) 3 Osteonecrosis is particularly well suited for the application of cementless technology, as the disease is typically localized to the femoral head (Fehrle etal, 1993, Kim etal, 1995). 4 In Juvenile inflammatory arthritis, the results of cemented fixation are not very good. Many series of cemented THA in JIA have shown a high rate of aseptic loosening. (Learmonth etal, 1989, Williams and McCullough, 1993) But uncemented implants are reported to have good results (Odent etal, 2005, 469) Disadvantages of uncemeted fixation (Taunton etal, 1997) 1 Intra-operative femoral fracures are more common with uncemented than with cemented components. 2 Thigh pain may adversely affect the outcome of even well-fixed uncemented components. Proximally coated stems which gain metaphyseal fixation are associated with a lower rate of thigh pain if the distal end is slotted to decrease stiffness. 3 The greater the diameter of the stem, the greater is the stiffening effect on the femur and the greater the degree of stress shielding. 4 Reports of poorer results in the elderly due to poor bone stock References 1 S T Canale (1998) Campbell's Operative Orthopaedics, ninth ed (St Louis, Mo: Mosby, 297-332. 2 Fitzpatrick, R., Morris, R., Shortall, E., and Murray, D. (1998). A structured review and modelling of outcomes of prostheses for primary total hip replacement surgery. Journal of Bone and Joint Surgery, British Volume, 81B, 273. 3 THA: Osteolysis (2007)," Wheeless' Textbook of Orthopaedics, http://www.ortho-u.net/o14/35.htm (accessed 23 Mar,) 4 Bloebaum, R., Mihalopoulus, N., Jensen, J., and Dorr, L. (1997). Postmortem analysis of bone ingrowth into porous-coated acetabular components. Journal of Bone and Joint Surgery, American Volume, 79A, 1013-22. 5 A A Hofmann et al (2000), "Cementless primary total hip arthroplasty with a tapered, proximally porous-coated titanium prosthesis: A 4- to 8-year retrospective review," Journal Arthroplasty 15 (October) 833-839 6 Geesink, R., de Groot, K., and Klein, C. (1987). Chemical implant fixation using hydroxyl-apatite coatings. Clinical Orthopaedics and Related Research, 225, 147-70. 7 S Rapp (2003), "Debate underscores pros, cons of using cementless stems in elderly," Orthopedics Today 23 (March) 14 8 Jasty, M., Bragdon, C., Burke, D., O'Conner, D., Lowenstein, J., and Harris, W. (1997). In vivo skeletal responses to porous-surfaced implants subjected to small induced motions. Journal of Bone and Joint Surgery, American Volume, 79A, 707-14. 9 Pilliar, R. (1983). Powder metal-made orthopedic implants with porous surface for fixation by tissue ingrowth. Clinical Orthopaedics and Related Research, 176, 42-51. 10 Morscher, E. (1992). Current status of acetabular fixation in primary total hip arthroplasty. Clinical Orthopaedics and Related Research, 274, 172-93 11 Murray, D., Carr, A., and Bulstrode, C. (1995). Which primary total hip Journal of Bone and Joint Surgery, British Volume, 77B, 520-7. 12 Engh, C. A.; Bobyn, J. D.; and Glassman (1987)., A. H.: Porous-coated hip replacement. The factors governing bone ingrowth, stress shielding, and clinical results. J. Bone and Joint Surg., 69-B(1): 45-55, 13 Sychterz CJ, Claus A, Engh C.( 2002). What We Have Learned About Long-Term Cementless Fixation From Autopsy Retrievals. Clinical Orthopedics and Related Research. Volume 405, December. 14 Bugbee, W., Culpepper, W., Engh, C., and Engh, C., Sr (1997). Long term clinical consquences of stress sheilding after total hip arthroplasty without cement. Journal of Bone and Joint Surgery, American Volume, 79A, 1007-12. 15 Maloney, W. J.; Sychterz, C. J.; Bragdon, C etal. (1996.).: Skeletal response to well-fixed femoral components inserted with and without cement. Clin. Orthop. 333: 15-26, 16 B E Bierbaum, K Howe (2000), "Total hip arthroplasty," Orthopedics Today (January) 6-7. 17 Kim YH, Oh JH, Oh SH (1995). Cementless total hip arthroplasty in patients with osteonecrosis of the femoral head. Clin Orthop;320:73-84. 18 Fehrle MJ, Callaghan JJ, Clark CR, et al. (1993) Uncemented THA in patients with avascular necrosis of the femoral head and previous bone grafting. J Arthroplasty;8:1-6. 19 Williams WW, Mc Cullough CJ (1993). Results of cemented total hip replacement in juvenile chronic arthritis. A radiological review. J Bone Joint Surg [Br].;75:872-874. 20 Learmonth ID, Heywood AW, Kaye J, et al (1989). Radiological loosening after cemented hip replacement for juvenile chronic arthritis. J Bone Joint Surg [Br].;71:209-212. 21 Odent T, Journeau P, Prier AM, etal (2005). Cementless Hip Arthroplasty in Juvenile Idiopathic Arthritis. Journal of Paediatric Orthopedics. Volume 25(4), July/August, pp 465-470 22 Taunton D, Culpepper WJ, Engh CA (1997). Treatment of complications in Primary Cementless Total Hip Arthroplasty. Clniical Orthopedics and Related Research. Number 344, 150-161 23 Whiteside LA, White SE, Engh CA, et al (1993.): Mechanical evaluation of cadaver retrieval specimens of cementless bone-ingrown total hip arthroplasty femoral components. J Arthroplasty 8:147-155 24 Engh CA, Hooten Jr JP, Zettl-Schaffer KF, et al (1995): Evaluation of bone ingrowth in proximally and extensively porous-coated anatomic medullary locking prostheses retrieved at autopsy. J Bone Joint Surg 77A:903-910. Read More