Evolution of Cemented Stems

Abstract

John Charnley was responsible for pioneering successful cemented hip arthroplasty. Changes in stem design were made in response to early complications such as stem fracture. Various philosophies of stem biomechanics emerged, namely stems performing in the taper slip mode and stems performing as a composite beam. Both stem designs may be successful, although it is important not to mix biomechanical philosophies. Later evolutions have occurred in response to surgical flexibility, specifically modularity and offset options. These attributes can impart new demands on the stem and, in some cases, retrograde developments have occurred. Cemented stems may yield excellent long-term results and have the potential to limit fixation to the proximal femur and establish a metaphyseal-loading regimen. The latest design of taper slip stems, such as the C-stem (DePuy Orthopaedics, Warsaw, Ind), may have advantages in maintaining proximal bone stock and preserving host bone. The historical developments and evolution of the cemented stem in total hip arthroplasty (THA) are discussed.

John Charnley is regarded as the pioneer of successful cemented total hip arthroplasty (THA). This article reviews the historical events in the development of cemented hip arthroplasty and traces the evolution of the cemented stem to the present. Knowledge of how biomechanics has developed, retrograde developments, and the philosophy behind current successful stem designs are also considered.

Historical Developments

It is not widely appreciated that Charnley experimented with a range of ideas in terms of design, fixation, and materials.^1-4 His initial experiments with resurfacing “double cup” arthroplasty using polytetrafluoroethylene for the socket and for a press-fit head failed due to complications with wear and femoral head necrosis (Figure 1). His attempts at using cementless and hybrid fixation and a press-fit, metal-backed socket also failed due to complications with fixation (Figure 2). His unfavorable experience with polytetrafluoroethylene led him to describe high-density polyethylene as a material for the socket (Figure 3).² Despite reduced wear, complications with fixation remained.

Figure 1: Polytetrafluoroethylene double cup prosthesis.

Figure 2: Metal-backed, uncemented high-density polyethylene socket.

Figure 3: High-density polyethylene socket.

Charnley’s close association with the dental school at the University of Manchester led him to believe that acrylic cement used in dental surgery may have applications in afixing a prosthesis to bone.⁴ Later, Charnley acknowledged that cement was fundamentally the most important element, which revolutionized total hip replacement (THR).

Realizing that a larger head increases frictional torque, Charnley finalized his total hip design — the cemented high-density polyethylene socket and the monoblock cemented femoral stem with head size of 7/8 inch or 22.225 mm. This was polished and manufactured out of EN58J stainless steel (Figure 4). The first cemented metal-on-polyethylene hip replacement was performed at Wrightington Hospital in England in November 1962.¹

Cemented hip replacements remain the gold standard for THR in terms of long-term clinical results.^5-10

Other Historical Developments

Figure 4: Low frictional torque cemented metal-on-polyethylene hip prosthesis.

At the same time, McKee and Watson-Farrar⁵ designed an artificial hip joint using a modified Thompson femoral head prosthesis and a chrome-cobalt metal socket with a metal-on-metal articulation, which was thought to allow fluid film lubrication and lead to better wear characteristics. However, the larger diameter ball had a high frictional resistance to movement and complications were encountered with socket loosening.

The Stanmore hip was first implanted in 1963. After several modifications, by 1970, it had a matte stem with a 25-mm head and an ultra-high molecular weight polyethylene cup.^11,12 Both components were cemented.

At a similar time, Muller⁹ developed a cemented prosthesis using a 32-mm diameter metal head with curved or a straight stem designs. The Muller SL (self locking) stem was based on the principle of achieving fixation in the bowed femur by inserting the largest possible stem.

Rationale for Evolution: Early Stem Fractures

Initial evolutionary changes occurred in response to early complications. In the following years, few complications or revisions were encountered until a fracture of the femoral stem appeared in the late 1960s.¹³

Although the overall incidence was low (<2%), Charnley was concerned and realized that the stem fracture was a result of cantilever bending. Proximal bending on a distally fixed stem resulted in stem deflection and fatigue fractures.^14-21

Charnley’s attempts to address the situation included changing the surface finish of the stem to a vaquasheened surface in 1969, which effectively surface-hardened the metal and modifying the stem design and geometry. The material was changed to 316 low carbon vacuum-melted stainless steel in 1971 to improve corrosion resistance and fatigue properties.

Figure 5: Evolution of the Charnley stem. From left to right: Original first-generation Charnley flatback, second-generation roundback stem, third-generation flanged Cobra stem, latest triple taper C-stem.

The second generation of roundback Charnley stems from 1973 had a rounded cross-sectional profile. In 1975, he introduced the third-generation Cobra flanged vaquasheened stem, which was intended to provide improved proximal loading. The offset was changed to 40 mm to decrease the bending moment. In 1982, the material was changed again to Ortron 90, a cold-worked stainless steel with high fatigue strength. The neck was reduced from 12.5 mm to 10 mm in 1984, which improved the head:neck ratio, range of movement, and impingement potential. These modifications were focused on improving the metallurgy, the size and strength of the stem, and the ability of the stem to load the proximal cement. In turn, the proximal femur provided proximal stem support and reduced cantilever failure.

When Charnley changed his stem from the flatback to the Cobra design, the biomechanical characteristics were changed from a tapered polished (force-closed) to a shape-closed or composite beam bio-mechanical design.¹⁷ Although this helped to overcome one complication, it may inadvertently have introduced another. Although the flatback stem had a higher incidence of stem fractures, the possibility for aseptic loosening was greater with the flanged stem as follow-up continued.^22,23 The biomechanical difference between the loading mechanisms of these two opposing philosophies was not appreciated at the time, but has been well described more recently in engineering principles (Figure 5).^17,19,22

The surgical technique also changed. The intramedullary bone block was introduced by Wroblewski et al²⁰ to occlude the femoral canal, allowing pressurization of cement and the tip of the stem to penetrate into the bone block effectively acting as a void. This would allow the stem to subside slightly without producing distal fixation and therefore improve proximal load transfer.

Exeter Experience

Different designs and philosophies were introduced during the same period. In many instances the biomechanical principles were understood after the stem was introduced. In 1969, Ling and Lee^16,18 introduced the collarless polished double tapered Exeter stem (Stryker, Newbury, United Kingdom). Initially, there was concern when the stem subsided within the cement mantle. However, it was later recognized that a relatively small amount of subsidence of 1 mm or 2 mm was not detrimental to long-term fixation. Proximal bone improved rather than deteriorated.

This leads to a better understanding of stem biomechanics and the viscoelastic behavior of cement (ie, creep). Cement continues to deform under constant load and, therefore, subsidence of the stem to engage the cement is beneficial rather than detrimental. Tighter wedging of the loaded taper resulted in compressive stresses within the cement mantle and reduced shear at the bone-cement interface. The nature of material and surface finish is critical with these stem designs. For the tapered stem to subside, a void is used at the distal tip of the stem that allows controlled subsidence unhampered by distal cement.^16-18

The original Exeter stems also had stem fractures. Hence, the 316 L stainless steel was introduced in 1976, and later Rex 734 (orthinox), which had higher fatigue strength. These newer stems had a matte surface due to a manufacturing change. This change had major complications, and high rates of aseptic loosening (10% at 10 years) were reported for the matte finished Exeter stem.

These complications were results of abrasion damage to the cement mantle caused by a relatively rough stem moving due to subsidence, generating large amounts of particulate cement and metallic debris, releasing cytokines,²⁴ and leading to osteolysis.

Another advantage of the taper polished stem was the tight seal produced by the subsiding stem that prevented the distal passage of particulate matter by fluid through the interface.²⁵ The polished surface was re-introduced in 1986, and the problem was resolved.

North American Perspective

Excellent long-term results with the original Charnley stem have been reported from several institutions.^26-30

The early experience of cemented stems was not entirely satisfactory in North America. In retrospect, this may have been due to stem design, head size, and cementing techniques. The term cement disease was used to describe cement-associated aggressive bone destruction seen with stem loosening.

Harris concentrated on improving cementing techniques in North America. Improved generations of cementation were described using distal and proximal occlusion of the femur, introducing cement with a gun, pulsatile lavage, pressurization, and porosity reduction. Harris et al^31,32 reported improved results using these techniques. Harris also argued in favor of a collar to strengthen the cement stem interface by pressurizing the cement, reducing component subsidence, and transferring load to the proximal femur.³³ An internal collar was an important feature of stem designs such as the Charnley, Muller, and McKee stems.

Because the cement stem interface was thought to be the weak link, attempts were made to strengthen this mechanical bond^34-42 and stem fixation by making it rough⁴³ to grip the cement or by precoating the stem with polymethylmethacrylate cement.³⁴ This philosophy was directly contrary to the Exeter principle.⁴⁴

However, several authors reported mechanical failures of these stems, beginning with cement prosthesis debonding especially in the polymethylmethacrylate precoated region, followed by rapid catastrophic failure characterized by gross loosening and proximal osteolysis.^30,39,45 Although not all precoated stems have resulted in failure,³⁴ many series have reported poor results with either rough stems⁴³ or precoated stems, namely the series from Iowa.

The grit-blasted, precoated Iowa component had a loosening of 24% at 8.2 years. These results were inferior to the results of Charnley flat-back polished stem reported by the same group, prompting the authors to switch back to the original polished surface Iowa stem.^40,41

Retrograde Evolution

In the 1990s, design changes and philosophies were introduced, many of which have not stood the test of time. The lessons learned from the Capital hip (3M Healthcare Ltd, Loughborough, United Kingdom) used mainly in the United Kingdom have been informative. This hip was based on the Charnley range, but some of the components were manufactured out of titanium and instruments were designed to create a narrow cement mantle around the stem. There was a high failure rate of the flanged modular titanium stem with debonding at the proximal stem-cement interface and early alarming osteolysis in the proximal femur, presumably as a result of relatively soft titanium abrading the acrylic cement and release of particulate debris.⁴⁵

However, other authors⁴⁶ have reported good short-term results with the Howse Mark II collarless double taper titanium alloy straight stems (Johnson & Johnson, Leeds, UK), which had a broad proximal profile, textured surface finish, inherent stability, and a thicker cement mantle. This illustrates the importance of material and surface finish when using stems that are likely to subside within the cement mantle.

Certain innocuous features have also been responsible for unexpected failures, such as the stem fractures, which were noticed in stems that had laser etching on the anterolateral tension side in one series.⁴⁷

It was also observed that restoration of the offset, particularly in young active patients, improved the abductor lever arm and reduced joint reaction forces. However, increasing the offset also increases rotational torque on the stem, which can lead to failure as a result of torsional instability of the stem within the cement mantle, if the stem design does not impart rotational stability.^48,49

Cement Mantle and French Paradox

The importance of the cement mantle is a topic of debate. The advantages of a complete cement mantle include reducing the access of any wear debris from the articulation to the bone-cement interface. Earlier work by Harris identified the potential for wear debris to track at the stem-cement interface and then flow through cracks in the cement mantle to the bone-cement interface releasing cytokines leading to osteolysis. The Exeter group also identified high pressures occurring in these areas. Fluid can flow between the stem and cement and small movements of the stem can generate high pressures. Focal osteolysis is the result of pressure, polyethylene debris, and cement and metallic debris. Therefore, it is desirable to have a complete cement mantle around the stem.⁵⁰ The optimum thickness of the cement mantle is thought to be 2 mm to 4 mm.

However, this conventional teaching has been challenged by the success of French-designed cemented stems, such as the Charnley Kerboull and the Ceraver Osteal. In France, the philosophy had been to insert the largest stem possible, which fully occupies the medullary canal, resulting in a thin or even deficient mantle. The rectangular cross-section provides intrinsic stability in torsion even in the absence of cement.⁵¹

Excellent long-term results were observed and it appears that if a stem is otherwise well fixed and does not migrate, then osteolysis will not occur. The intrinsic stability protects the thin mantle. The tight fit of the stem is thought to immobilize the cement interface by eliminating micromotion during the critical time of setting. Also insertion of a tight canal-filling stem vigorously into doughy cement produces better pressurization and stronger mechanical interlock.⁵²

This is further explained by Shen¹⁷ who postulated that stems can be either “shape closed” (stick up and stay) or “force closed” (taper slip) devices. In the stick up and stay design, the stems can be relatively rough and in some circumstances made out of titanium and will function well without osteolysis. In the taper slip mode, in which the stem subsides within a cement mantle, it is desirable to have a relatively hard material such as stainless steel or cobalt chrome and highly polished surface to reduce abrasion.²⁵ Perhaps one of the most important lessons learned over the past decade was that rough stems made out of relatively soft materials suspended in a cement mantle are vulnerable to osteolysis and failure.

Both of the biomechanical philosophies (shape closed and force closed) can be successful and the majority of the stems with excellent 10-year survivorship in the Swedish Register are essentially shape-closed designs and the Spectron (Smith-Nephew, Memphis, Tenn) and Lubinus (Link, Kiel, Germany) are relatively rough stems.²⁴ However, the main advantage of using a taper slip stem is to load the proximal femur, and a third taper along the medial edge, as in the C-stem (DePuy), may further enhance proximal loading.

Factors in Cemented Stem Design

Various factors are important in the design and manufacturing of modern cemented femoral components.

Materials and Cross-Sectional Geometry
Stiff materials (Co-Cr and stainless steel) decrease cement stresses, are harder, and are more abrasion resistant. Sharp corners were undesirable. Broad medial borders without sharp corners reduce cement strain proximally and broad lateral borders enhance component strength.^52-54

Implant Stability
Axial and torsional stability of the femoral component in the cement mantle are important for long-term survival. Canal-filling rectangular stems are inherently stable with torsional loads and in the absence of cement. These fit and fill stems, such as the Charnley-Kerboull and the Ceraver Osteal, have excellent long-term results.⁵¹

Offset
An optimal offset can improve abductor mechanics and prevent medial impingement, but higher offsets result in higher torsional loads in a stem with unfavorable cross-sectional geometry. Modern designs incorporate optimal intrinsic stability to better tolerate the torsional load.^48,49,55

Collar
Taper slip designs, which require controlled subsidence, are collarless with a highly polished surface. Other designs such as the Charnley-Kerboull have a double-taper geometry and also a collar. Collared and collarless implants have been successful long-term.^33,44,53

Centralization
An adequate cement mantle circumferentially around the prosthesis is beneficial for load transfer from the stem to the cement and for prevention of transport of wear debris by fluid through cement cracks. Cement mantle defects in association with rough surface finish have been associated with stem failure due to aseptic loosening. Use of a centralizer on the stem may help to avoid mantle defects by ensuring a uniform mantle.^56,57

Surgical Technique
While preparing the femoral canal, broaching alone seems to have potential advantages over reaming and broaching, as it tends to leave more bone stock behind to improve cement bone interlock. Cleansing of bone with pulsatile lavage and brush, followed by pressurization of cement to achieve good penetration, is an important factor in achieving good long-term results.⁵⁸

Type of Cement
Different types of cement used with the same Charnley hip prosthesis have shown important differences in failure rates. Low viscosity cement showed increased risk of failure compared to high viscosity cement as reported by the Swedish Registry. Unfavorable results were also reported with Boneloc cement using the Charnley stem.^24,59

Preheating
Preheating of the stem to 50° C reduces the porosity of cement and improves shear strength and fixation at the stem cement interface.⁶⁰

Surface Finish
Surface finish has incited much controversy in the design of the modern cemented stem. The original Charnley flat back polished stem had an average roughness (Ra) of 0.1 mm. Designers have also developed matte finishes with Ra from 0.6 mm to 0.75 mm, and grit-blasted finishes with Ra of 1.5 mm and above.^61-63

Although a rougher surface can provide adhesion and mechanical interlock with cement, it causes abrasive damage to the cement mantle, in the presence of stem instability and generates larger amounts of particulate matter. Furthermore, movement of a debonded stem with a rough surface can provide a preferential channel for fluid migration and transport of wear particles to the distal part of the implant leading to osteo-lysis within areas of mantle defects.^50,64

The polished surface of a taper slip stem reduces the abrasive damage to the cement mantle while subsiding, and tighter wedging within the cement also prevents fluid transport due to a tight seal at the stem cement interface.⁵⁰ Modern tapered collarless prostheses such as the Exeter and the C-stem have a highly polished surface.

Taper
A tapered geometry from proximal to distal is designed to allow controlled subsidence of the stem within the cement mantle, permitted by the tensile stress relaxation of the cement, to a new position of stability. A polished surface finish is important in the success of these taper slip stems.^65-68

Need for the Third Taper
Proximal femoral strain shielding was identified as a long-term complication with the Charnley stem, which arose because of the load transfer by a distally supported stem that was no longer subject to fracture. Because it was appreciated that loading of the proximal and medial femur is critical for load transfer, a third taper was added by Wroblewski et al,⁶⁹ in the axial plane from lateral to medial, to develop the C-stem at Wrightington.

The triple taper C-stem has shown positive bone remodeling in the proximal femur in the short-term due to beneficial loading of the medial proximal femur. The shape provides resistance to torsional displacement and the rounded edges avoid sharp corners and cement cracking from stress.⁶⁹

Much has been learned over the past four and a half decades. A complex interaction is found between mechanics, geometry, and surface finish. The taper slip and composite beam stem philosophies have produced good long-term results. Excellent results have been reported from the Swedish registry for several cemented stems including the Charnley, Exeter, Lubinus, and Spectron stems.²⁴ It is important to understand these philosophies and avoid mixing stem designs. There is a trend in the United Kingdom toward a tapered polished stem as the cemented option for THA.

Proximal loading of the femur is beneficial, and therefore we currently use the triple tapered C-stem as the cemented stem option at out institution. Well-designed, well-implanted cemented stems can give exceptional long-term results in patients of all ages. Biomechanical knowledge acquired over the past 40 years has helped surgeons understand what works well and how potential failures may be avoided. At Wrightington, where John Charnley pioneered cemented hip replacements, we continue to use cemented stem fixation in all age groups.

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Authors

From the Centre for Hip Surgery, Wrightington Hospital, Lancashire, United Kingdom.