Recent advances in designs, approaches and materials in total knee replacement: literature review and evidence today.
Abstract: Ever since Themistocles Gluck described the use of an ivory cup as a tibial hemiarthroplasty in 1894, knee arthoplasty has continued to evolve. Both human ingenuity and intensive clinical research has led to an improved understanding of biomaterials and knee kinematics, resulting in the modern total knee replacement which has enjoyed such a clinical and commercial success. As it increases in popularity, attempts to improve knee arthroplasty have been driven by demands for improved function and implant survival, particularly in younger, more demanding patients. Research continues to see if advances in implant instrumentation, materials and design will translate into improved clinical outcomes and longevity.

KEYWORDS Knee/Arthroplasty/Advances/Materials
Subject: Orthopedic equipment and supplies (Usage)
Authors: Wong, James M.
Khan, Wasim S.
Chimutengwende-Gordon, Mukai
Dowd, George S.E.
Pub Date: 05/01/2011
Publication: Name: Journal of Perioperative Practice Publisher: Association for Perioperative Practice Audience: Academic Format: Magazine/Journal Subject: Health; Health care industry Copyright: COPYRIGHT 2011 Association for Perioperative Practice ISSN: 1750-4589
Issue: Date: May, 2011 Source Volume: 21 Source Issue: 5
Topic: Canadian Subject Form: Orthopaedic equipment
Product: Product Code: 3842111 Orthopedic Appliances; 3842110 Orthopedic Appliances & Supplies NAICS Code: 339113 Surgical Appliance and Supplies Manufacturing SIC Code: 3842 Surgical appliances and supplies
Geographic: Geographic Scope: United Kingdom Geographic Code: 4EUUK United Kingdom
Accession Number: 272168018
Full Text: Since the development of modern total knee arthroplasty implants in the 1970s, total knee replacements have been overwhelmingly successful in their primary function of abolishing pain in degenerative joints (Ritter et al 2001). Modern implants enjoy a high satisfaction rate and an acceptably low revision rate (Noble et al 2006, Bourne et al 2010). This success can account for their popularity, with the 7th National Joint Registry (NJR) report documenting 79,263 primary total knee arthroplasties performed in the England and Wales alone in 2009-2010 (NJR 2010). This demand is expected to expand, with some estimates from the United States predicting a 673% increase by the year 2030 (Kurtz et al 2007).

With an increasing incidence of early onset degenerative, disabling rheumatoid and post traumatic arthritis total knee replacements are now being offered to patients at a younger age. The continued evolution of knee arthroplasty has been driven by the need for longer lasting implants for these patients, with both the latest NJR report (NJR 2010) and the Swedish Knee Arthoplasty Register (SKAR 2010) documenting higher revision rates in younger patients. However, some of these changes are undoubtedly market driven, with a specific procedure often marketed directly at patients. Developments in knee arthroplasty have explored the possibilities of new implant designs, new surgical techniques and new implant materials and these are discussed below.

New total knee replacement (TKR) designs

The vast majority of total knee arthroplasty implants in use today are versions of the 'condylar' knee replacements developed in the 1970s, widely credited to work by Freeman and Swanson of the London Hospital and Imperial College, and Insall, Burstein and Walker of the Hospital for Special Surgery in New York (Shetty et al 2003). These advances in design have resulted in a marked improvement in results and longevity.

Condylar knee replacements incorporate an anatomically shaped femoral component, usually made from a cobalt-chrome alloy, with an anterior flange to articulate with the patella. The femoral component articulates with a polyethylene tibial tray, which is usually attached to a titanium tibial component (Figure 1). Inserting these implants involves resection of the bone ends to create parallel and equal spaces in flexion and extension, and then balancing the soft tissues to convey stability. These implants are normally cemented in place using acrylic polymethylmethacrylate cement.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Alterations to this design in recent years include modifications to improve flexion and altering the implant's dimensions based on the patient's gender.

The high flexion knee

Postoperative flexion is one of the parameters that surgeons and patients use to evaluate the success of total knee arthroplasty. Whilst the human knee is capable of 150 degrees of flexion, patients after total knee arthroplasty rarely achieve more than 110 degrees. Attempts have therefore been made to modify prostheses to allow more flexion, particularly in the younger patient. This can be achieved by increased 'wraparound' of the femoral component around the posterior femoral condyles (Figure 2). This theoretically enables flexion of the prosthetic knee in excess of 130 degrees, whilst maintaining the same performance at normal ranges of flexion.

Several high-flexion prostheses are now available, with varied results in the published literature. Although some randomised, prospective studies have shown that high flexion designs achieve 1013 degrees more flexion than standard knee arthroplasties (Huang et al 2005, Weeden and Schmidt 2007), others have shown no difference in eventual flexion (Kim et al 2005, Mehin et al 2010). Some have questioned the need for these implants. One study investigated knee movements after high flex TKR, and showed that flexion greater than 90 degrees was needed for only 0.5% of the time, and flexion of 120 degrees was needed for less than 0.1% of the time during activities of daily living (Huddleston et al 2009). Of some concern is the finding that high flexion activities induce higher implant stress levels, raising the possibility that implant longevity may be compromised (Barink et al 2008). Long term results regarding flexion and implant survival are clearly needed.

Gender specific designs

Small but significant anatomical variations in male and female knees have long been recognised. Measurements taken intraoperatively suggest that women have narrower femoral condyles than men (Chin et al 2002). This can make sizing of femoral components, which is typically based on the anteroposterior dimensions of the femur, awkward. An appropriately sized femoral component may be too wide, altering collateral ligament balancing. This may have to be accepted, or alternatively a smaller femoral component may be used, resulting in laxity in flexion. Women also differ from men in that they have a different orientation of the articulation between the femur and the patella.

Manufacturers have now introduced gender specific total knee arthroplasty designs with modified dimensions to accommodate the anatomical differences between sexes. Clinical results are starting to appear in the medical literature. In a prospective randomised trial comparing gender specific with standard implants, Kim et al (2010a) found no significant differences between the two groups in terms of satisfaction, clinical or radiological outcome.

The medial pivot knee

Attempts have also been made to adapt knee arthroplasty designs as our understanding of knee kinematics changes. Work by Freeman using MRI studies of cadaveric knees have demonstrated that, whilst the lateral femoral condyle moves posteriorly on the tibia up to 2cm when the knee flexes, the position of the medial femoral condyle hardly moves, pivoting like a ball and socket joint (Freeman 2001). The medial rotation knee (Wright Medical Technology, Arlington, Tennessee, USA) was designed to reproduce this movement. This design has a spherically shaped medial femoral condyle which articulates with a matched, conforming tibial component. Although this represents an increase in constraint, and potentially increases the forces being transmitted through the tibial component, early results have not shown an increase in loosening (Amin et al 2008). Results after five years have shown 99% survival of the implant (Karachalios et al 2009), and longer term results are now awaited.

New surgical approaches

Minimally invasive surgery

Minimally invasive surgery (MIS) represents an alteration to the standard medial parapatellar approach to the knee used in 92% of knee arthroplasties (NJR 2010). It is defined as a minimally invasive technique in which the incision and surgical dissection have been modified to reduce surgical morbidity (Malik & Dorr 2007). Several approaches have been described, all aiming to reduce size of the skin incision, but also reducing the amount of soft tissue dissection, conserving the subvastus insertion and not everting the patella. New instrumentation has been introduced to allow joint arthroplasty through this more limited exposure, without causing trauma to tissue due to excessive traction. A worrying development is that MIS appears to have been marketed directly at patients eager for a better cosmetic result.

Advocates of MIS feel that it represents a natural evolution in the approach to knee arthroplasty, limiting soft-tissue disruption, and therefore some of the pain and functional delays in recovery observed postoperatively. Detractors feel that the more limited exposure may result in component malposition (Dalury et al 2008) and hence increased complication rates. The debate regarding the role of MIS has not been settled by the medical literature, with studies for total knee arthroplasty reporting a wide range of clinical results. Some have shown early benefits, such as decreased pain, shorter functional recovery, and improved range of knee flexion (Haas et al 2004, Laskin 2005, Varela-Egocheaga et al 2010). Others, however, have failed to demonstrate any substantial difference between MIS and conventional approaches in the postoperative period (Seon & Song 2006, Kolisek et al 2007, Wulker et al 2010).

Navigation

Although excellent results have been reported with modern implant designs, malpositioning of implants can contribute significantly to early failure (Ritter et al 1994). Most knee systems utilise a combination of intramedullary and extramedullary instrumentation together with extensive visual referencing to guide appropriate resection of bone. Variations in anatomy, difficulty with exposure, and bone loss can affect the accuracy of conventional instrumentation and compromise postoperative limb and component alignment. Some studies have suggested that component malposition greater than 3 degrees can occur in up to 10% of knee replacements using conventional jigs (Stulberg et al 2002). Rotational malalignment in particular is a significant problem, and often difficult to identify postoperatively. Computer assisted surgery, or navigation, has been introduced in an attempt to improve the accuracy of bone cuts and implant placement.

Several navigation systems are now available. These systems utilise preoperative CT scans, intraoperative fluoroscopy or intraoperative digital registration to determine the location and orientation of multiple anatomical landmarks. Many systems utilise pins with reflective trackers drilled into the femur and tibia. The positions of these trackers are monitored by infrared cameras. These data are used to calculate the mechanical and anatomical axes of the limb, and then guide the appropriate position and orientation of bone cuts. Many of these systems are designed to coincide with MIS approaches. Despite improvements, many find these systems cumbersome, and time consuming to use (Dutton et al 2008). There are also concerns regarding cost. Multiple prospective randomised controlled trials (Matziolis et al 2007, Mullaji et al 2007, Dutton et al 2008), and more recently metaanalyses (Mason et al 2007) have shown increased accuracy in bone cuts when compared to conventional jigs, achieving accuracy to within 1 degree in the coronal and sagittal planes, and showing improved accuracy in rotational alignment (Chauhan et al 2004). Whether or not this increased accuracy will translate into improved function and implant survival, however, remains to be seen (Bauwens et al 2007). One study has already pointed out that accurate bone cuts do not guarantee reproducible implant positioning, recording inaccuracies related to implant insertion after accurate bone cuts (Catani et al 2008).

Many feel that navigated systems already have a place in knee replacement surgery, particularly in the difficult patient where tibial or femoral deformities prevent the accurate use of conventional jigs. Whether or not these systems will gain widespread acceptance for more straightforward procedures depends on their ongoing evolution, especially regarding their expense as well as their speed and ease of use.

New arthroplasty materials

In the early 1900s, surgeons experimented with interposition arthroplasty in the knee as a means of treating arthritis whilst preserving movement. Materials used were initially biological, including pieces of pig bladder, fascia lata and skin flaps. Nonbiological implants made of cellophane and nylon were also tried, eventually giving way to metallic implants in the 1940s. The composition of knee arthroplasty implants continued to evolve as knowledge of biomaterials and their properties improved. The vast majority of modern implants incorporate a cobalt-chrome alloy femoral implant articulating with a polyethylene tibial tray, usually attached to a titanium tibial component. The continued improvement of these materials remains the subject of intense research.

Improving polyethylene

Ultra-high molecular weight polyethylene (UHWPE) was first introduced as a bearing surface for joint arthroplasty by Sir John Charnley over 40 years ago. Its strength, wear properties, low coefficient of friction and the ability to mould or machine it into a desired shape have contributed to its success. Knee replacements normally fail due to fatigue and delamination of polyethylene, but implant failure can also be caused by the generation of polyethylene wear particles. These particles are biologically active, and induce an inflammatory response when phagocytosed by macrophages. The degree of biological activity is dependent on the amount, size and shape of the wear particles generated. Inflammatory mediators released by macrophages stimulate osteoclasts to resorb bone, causing osteolysis, loosening and ultimately failure of the prosthesis. Polyethylene has therefore, despite its clinical success, been the subject of much research to improve its wear properties to reduce osteolysis. Attempts to improve performance by reinforcing UHWPE with carbon fibre, and the introduction of UHWPE with higher crystallinity (Ries et al 1996) both failed clinically, despite considerable in vitro testing.

More promising in the evolution of polyethylene is the changes made to sterilisation via gamma irradiation, which increases the cross-linking between polyethylene polymer chains, producing highly cross-linked polyethylene (HXLPE). This improves the wear properties of the polyethylene, but at the cost of generating free radicals which induce an oxidative reaction, making the polyethylene brittle. Heating HXLPE stabilises it against oxidisation but alters its mechanical properties, notably reducing strength and fatigue resistance. Multiple manufacturers have produced HXLPE implants, all differing in their irradiation and thermal stabilisation regimes. This first generation of HXLPE implants have shown decreased wear rates when used in hip arthroplasty after 4-5 years (Dorr et al 2005, Manning et al 2005, Geerdink et al 2006, Engh et al 2006), with one study showing a 95% reduction in wear (Digas et al 2007). Some failures have occurred, however, due to fractures of tibial polyethylene posts in knee arthroplasty (Sharkey et al 2002). Further work is ongoing in the generation of so-called second generation HXLPE, aiming to further reduce oxidation through multiple cycles of irradiation and thermal stabilisation treatments. Thus far in vitro studies have suggested a further reduction in wear rates (Tsukamoto et al 2008).

Another strategy to reduce HXLPE oxidation is adding the antioxidant alpha-tocopherol (vitamin E) to the polyethylene, avoiding thermal stabilisation treatments which can alter its mechanical properties. This has been shown to improve the fatigue crack propagation resistance of polyethylene (Oral et al 2008) without compromising wear resistance in vitro (Wannomae et al 2010). There are as yet no clinical trials published in the medical literature.

It should be noted that there are some concerns regarding the application of HXLPE to knee arthoplasty. Although joint simulator studies have shown decreased wear, the polyethylene wear particles generated are smaller than those produced from conventional polyethylene (Fisher et al 2004) with evidence that these particles may be more biologically active (Illgen et al 2008), with a greater potential to cause macrophage-mediated osteolysis and loosening.

Oxinium

Although numerous attempts have been made to decrease polyethylene wear, and thereby increase the longevity of knee implants, most of these have centred on changes in polyethylene composition and implant design. A new approach has been to alter the composition of the surface articulating with the polyethylene (the femoral component). Ceramic components are much harder than metals with a lower coefficient of friction and better scratch profile. Ceramics have been used as bearing surfaces in hip replacements, with greatly reduced wear, but their brittleness makes them unsuitable for use in knee arthroplasty.

A new metal alloy has been introduced for the manufacture of femoral components in knee arthroplasty as an alternative to the more conventional cobalt chrome. This oxidised zirconium, also called oxinium (Smith and Nephew, Memphis, TN) is produced via a thermally driven oxygen diffusion process which converts the surface of the metallic zirconium alloy into a durable oxide with a low friction coefficient. This surface is essentially a ceramic, conveying excellent wear properties, but the remainder of the prosthesis remains a metal alloy, removing the fracture risk.

[FIGURE 3 OMITTED]

In vitro studies regarding oxinium knee components in wear simulators (Tsukamoto et al 2006, Lee et al 2009) have shown improved scratch resistance and decreased polyethylene wear when compared to standard implants. Clinical trials have shown good results at two years (Laskin 2003) and five years (Innocenti et al 2010). Whether or not the theoretical advantage of using oxinium implants is translated to improved long term clinical outcome remains to be seen. Studies comparing samples of synovial fluid from total knee replacements using oxinium and cobalt chrome have shown no difference in the weight, size or shape of generated polyethylene wear particles (Kim et al 2010b), suggesting that rates of osteolysis and loosening may be unchanged.

Uncemented knee replacements

Fixation of implants with acrylic polymethylmethacrylate cement is currently the gold standard in total knee replacement, and is used in over 92% of total knee replacements (NJR 2010). In contrast, surgeons have been reluctant to utilise cementless fixation due to past failures in the early generations of cementless knee arthroplasty designs. With knee replacements being considered in younger patients, however, cementless fixation has enjoyed a renewal of interest as a potentially more durable mode of fixation. This has been aided by the availability of improved biomaterials, including highly porous tantalum and titanium biomaterials with morphologies and mechanical properties resembling native trabecular bone, encouraging new bone to grow into the implant (Figure 3). This may provide faster and better osseointegration than previous cementless fixation and is already popular in revision hip surgery. Early results have been encouraging, with clinical trials of a tantalum trabecular metal tibial tray (Zimmer, Warsaw, IN) reporting three year results equivalent to cemented prostheses (Dunbar et al 2009, Helm et al 2009).

Conclusion

Knee arthroplasty continues to evolve, with efforts to improve postoperative function and implant survival subjecting almost all aspects of the status quo to continued research and clinical trials. When reviewing these new developments it is worth considering that existing condylar knee replacement designs, utilising a cobalt chrome femoral component fixed with polymethylmethacrylate cement and articulating with a UHMWPE tibial tray, are already hugely successful, achieving excellent clinical results, high patient satisfaction and low revision rates. Caution must be exercised to ensure that a theoretical advantage does not result in a poorer clinical outcome. Orthopaedics has seen many good implants ruined by 'improved' engineering.

Task 1

Explore your department and identify how many different manufacturers' implants you have available on the shelf for primary knee replacements. Who decides on what implants are used and what are the decisions based on?

Notional Learning Hours

Knowledge and Skills Dimension

Core 3: Health, safety and security

HWB9: Equipment & devices to meet health and well-being needs

G1: Learning and development

Task 2

What things can you do intraoperatively in theatre to reduce the chances of a patient undergoing a total knee replacement getting an infection?

Notional Learning Hours

Knowledge and Skills Dimension

HWB6: Assessment and treatment planning

HWB7: Interventions and treatment

HWB9: Equipment & devices to meet health and well-being needs

G1: Learning and development

Task 3

Developments in hip and knee replacement surgery share common themes. Read the article:

Dheerendra et al 2010 Recent developments in total hip replcements: cementation, articulation, minimal-invasion and navigation Journal of Perioperative Practice 20(4) 133-138

Notional Learning Hours 1 hour

Knowledge and Skills Dimension

HWB6: Assessment and treatment planning

HWB7: Interventions and treatment

HWB9: Equipment & devices to meet health and well-being needs

G1: Learning and development

Task 4

What is your understanding of 'Navigation' and 'Minimal Invasive Surgery'? What are the advantages and disadvantages?

Notional Learning Hours 30 mins

Knowledge and Skills Dimension

Core 3: Health, safety and security

Core 4: Service improvement

Core 5: Quality

HWB2: Assessment and care planning to meet people's health and well-being needs

HWB7: Interventions and treatment

No competing interests declared

Provenance and Peer review: Unsolicited contribution; Peer reviewed; Accepted for publication February 2011.

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Correspondence address: Wasim Khan, UCL Institute of Orthopaedics and Musculoskeletal Sciences, Royal National Orthopaedic Hospital, Stanmore, Middlesex, HA7 4LP. Email: wasimkhan@doctors.org.uk

About the authors

James M Wong MBBS, FRCS (Trauma and Orthopaedics)

Specialist Registrar, Trauma and Orthopaedics, Royal Free Hospital, London

Wasim S Khan MBChB, MSc, MRCS, PhD

Clinical Lecturer, University College London, Institute of Orthopaedics and Musculoskeletal Sciences, Royal National Orthopaedic Hospital, Stanmore

Mukai Chimutengwende-Gordon MBChB, MSc, MRCS

Academic Orthopaedic Registrar, University College London, Institute of Orthopaedics and Musculoskeletal Sciences, Royal National Orthopaedic Hospital, Stanmore

George S E Dowd MD, MCh, FRCS

Consultant Orthopaedic Surgeon, Royal Free Hospital, London Director of Knee Unit, Wellington Hospital, London
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