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Optimizing the Femoral Component Cement Mantle in Total Hip Arthroplasty

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Abstract

Aseptic loosening is a common cause of long-term failure of cemented femoral components in hip arthroplasty. Initiation of aseptic loosening has been associated with suboptimal cement mantle thickness and uniformity with the resultant progressive development of detrimental cement mantle defects. Long-term success is highly dependent on maintaining and protecting the integrity of the cement mantle and its interfaces primarily by decreasing cement mantle stresses. High cement stresses that initiate debonding and cement fracture can be controlled and minimized through the use of various surgical techniques that assist in creating an optimally thick, symmetric, and homogeneous cement mantle.

Progressive aseptic loosening is a common cause of long-term failure of cemented femoral components in total hip arthroplasty (THA).^1-4 Initiation of aseptic loosening has commonly been associated with the presence of suboptimal cement mantle thickness and uniformity with the resultant progressive development of further detrimental cement mantle defects.² Changes in cement technique have been developed to improve outcomes after cemented THA. First-generation cementing techniques that included finger packing of hand-mixed cement into an “unprepared, unplugged” femoral canal were first improved with the incorporation of a distal canal plug, preparation of the canal using brushes, pulsatile lavage and drying techniques, and retrograde filling of the canal using a cement gun. Third-generation cementation improvements included vacuum mixing or centrifugation of cement to reduce inclusions, pressurization of the cement column, and the use of stem centralizers to facilitate creation of a cement mantle with uniform thickness. These improvements in cement and canal preparation and cement insertion techniques were designed to enhance outcomes primarily by improving fixation at the cement-bone interface.^1,2,5-10

Although these enhancements have improved outcomes, finite element, laboratory, and clinical studies have suggested the stem-cement interface is the “weak link” in the cemented femoral component construct.^6,9-15 The early development of stem-cement interface debonding (separation) and subsequent cement fracture are believed to be the initiating events of aseptic loosening.^5,14,16 Research analyses suggest that these initiating events result from cement failure due to stresses experienced at the cement mantle that exceed the fatigue endurance limit of both the stem-cement interface and the cement material itself.^1,9-13,15 Further studies have shown, however, that the high cement stresses that initiate debonding and cement fracture can be controlled and minimized through attention to various methods and techniques that assist in the creation of an optimally thick, symmetric, and homogeneous cement mantle. It has become apparent that the long-term success of cemented THA is highly dependent on maintaining and protecting the integrity of the cement mantle and its interfaces primarily by decreasing cement mantle stresses.^13,17

Cement Mantle Stresses

Weight bearing stresses experienced in the native femur are fairly uniform. After cemented THA, normal femoral stresses are permanently altered due to transfer of loads through the femoral component and cement mantle.⁸ As a result, the native femur, femoral component, and cement mantle are subjected to individual loads producing various stresses in predictable locations.^{6-8,11,15,16,18-20} Stresses experienced in the cement mantle have been shown by multiple finite element, laboratory, and clinical analyses to be highest at the stem tip and secondarily at the proximal-medial cement mantle.^5-8,11,15-20 In addition, finite element analysis has determined that the cement mantle stresses present in the region of the stem tip are highest in the cement immediately adjacent to the implant (stem-cement interface), where good fixation is critical to avoid development of the initiating elements of eventual implant loosening.⁶

Cement mantle stresses can be significantly affected by many factors including mantle thickness, uniformity, and homogeneity. Stem malalignment produces non-uniform cement mantle thickness in key areas. Defects or voids in the cement mantle effectively reduce bulk cement thickness and can have a substantial effect on cement stresses. Variations in implant geometry (eg, diameter and contour) and material also have been shown to affect stresses experienced in the cement mantle. Optimization of each of these factors is essential to obtaining a quality cement mantle and avoiding potentially devastating consequences associated with elevated cement mantle strains.

Consequences of Cement Mantle Stresses

As previously stated, debonding at the prosthesis-cement interval is believed to be the earliest manifestation of cemented femoral component failure and has been directly associated with areas of deficient or thin cement mantles.^{8,10,12,13,15,21} This phenomenon occurs relatively early after cemented THA and is seen in nearly all retrieved specimens, but is uncommonly observed on early radiographs.^8,16,21 Finite element and clinical studies have shown that debonding occurs most commonly in areas experiencing high cement mantle stresses, typically beginning at the distal stem tip, secondarily at the proximal-medial cement mantle and ultimately progressing to the mid-stem levels.^{10,12,13,18,20-22} Debonding has also been shown to further increase stresses experienced at the adjacent stem-cement interface,^2,10,13,21 although the effect of these increased stresses is debatable. Evaluation of retrieved specimens has suggested that early debonding accelerates further debonding,²¹ whereas finite element analysis suggests that an acceleration of debonding typically does not occur until >50% of the stem-cement interface has debonded.¹⁰ Increased stresses causing and resulting from debonding have been directly associated with the subsequent development of cement mantle fractures.^13,21

The development of microscopic cement fractures, typically originating at the stem-cement interface, have also been associated with sections of the cement mantle <1 mm in thickness.^7-9,14,16,21 As with cement debonding, cement fractures have been shown in both laboratory and clinical studies to occur in up to 100% of retrieved implants, typically developing early following cemented stem implantation and rarely seen on short-term follow-up radiographs.^8,16,21 These cement mantle fractures may progress over time and extend through the entire width of the cement mantle.^8,21

Finite element analysis has demonstrated that cement fractures associated with thinner cement mantles extend faster through the bulk cement to the cement-bone interface and bony substance.⁴ Consequently, a pathway is created for potential debris generated at the stem-cement interval to migrate to the bone and potentially initiate the histiocytic responses associated with osteolysis.^16,21,23-25

Aseptic femoral component loosening is believed to be a late event following the earlier mechanical changes (ie, cement debonding and fracture) typically associated with thin cement mantles and subsequent compromise of the cement interactions with the prosthesis and bone.^7,19,21,26 It is postulated that late symptomatic component loosening occurring at the bone-cement interval is initiated by osteolytic reactions resulting from migratory debris generated at a compromised stem-cement interface.^13,21,23 Clinical studies have demonstrated that areas of osteolysis seen around cemented femoral components are frequently associated with thin cement mantles (<1 mm) where focal cement mantle fractures can be expected.^{4,14,16,17,21,25}

Optimization of the Cement Mantle

Many techniques optimize the quality of the cement mantle. Improvements in the inherent properties of the cement (increased strength, reduced brittleness, improved interface adherence, etc.) to increase strain resistance and thus retard early debonding and microfractures will aid in the quality and long-term success of the cement.^10,17

Canal preparation using appropriately sized broaches to allow a mantle of adequate thickness, pulsatile lavage, and brushing and drying of the prepared canal before and during insertion will enhance the quality of the mantle and bone-cement interface.^1,9

Cement preparation using centrifugation or vacuum mixing to minimize pore formation and timing of cement injection to achieve optimal viscosity during insertion will additionally improve the ultimate quality of the cement mantle.^9,21,27 Occlusion of the canal using a distal plug, retrograde filling of the canal and cement gun pressurization of the cement column with a tight proximal seal are essential in achieving an interdigitating, uniform, and homogeneous cement mantle.⁹

Finally, attempts to clear both the medial cancellous bone from the calcar region and clearance of the posterior calcar to permit straight insertion of the femoral component down the curved femur have been recommended.²³ Achievement of a high-quality cement mantle radiographically has been associated with increased durability of cemented femoral component fixation when compared to radiographically poor-quality cement mantles.^2,8

Although finite element and clinical evaluations have determined that cement thickness is a primary factor in the generation of cement mantle stresses,^{5,6,18,19,22,28,29} exact recommendations for optimal cement thickness vary. Proximal-lateral cement stresses, regardless of cement thickness, have not been directly implicated as a cause of ultimate component loosening and thus the optimal proximal cement mantle is not of uniform thickness medially vs. laterally.⁸

Clinical studies have shown that a proximal-medial cement mantle >10 mm in thickness has been associated with a significant increase in cement fracture, radiolucent lines at the prosthesis-cement and bone-cement intervals, and progressive component loosening when compared to proximal-medial cement mantles that measured 2-5 mm in thickness.¹⁹ These analyses have further shown that preserving <2 mm of proximal-medial cancellous bone for 30 mm distal to the femoral neck cut increased cement mantle thickness and reduced proximal-medial cement strains, the incidence of cement fractures, and the incidence of progressive implant loosening when compared to those cases in which >2- 5 mm of proximal-medial cancellous bone was retained.^1,11,19,21

Recommendations based on percent of intramedullary canal fill by the femoral component have suggested that minimal cement strains are generated when the stem occupies 80%-90% of the proximal femoral canal.¹⁷ Although femoral anatomy and stem geometry may dictate to some extent the space available for the cement mantle, creation of a cement mantle <2 mm has been associated with significantly elevated cement stresses. Various finite element, laboratory, and clinical studies have strongly recommended using techniques (ie, use of smaller stems in medial-lateral dimension, lateralization of the proximal stem, avoiding varus implantation, etc.) that ensure a proximal-medial cement thickness of 2-6 mm.^{3,11,15,19,20,30}

Because cement stresses have been shown to increase proximally to distally,^7,8,15 distal cement deficiencies (<1 mm) near the stem tip substantially increase the ultimate risk of implant failure.²⁶ Numerous studies have observed that a higher incidence of distal cement fractures and implant failure occurs with distal cement mantles <2 mm thick.^2,7,28 In contrast to the nonuniform proximal-medial cement mantle, finite element, laboratory, and clinical analyses have determined the importance of obtaining a uniform distal cement mantle thickness of more than 2-5 mm.^{6,3,7,15,22,28-30} An additional finite element analysis has demonstrated that extending the distal cement mantle >7.5 mm distal to the stem tip provides no significant additional decreases in distal cement strains.⁶

Malalignment due to inadequate centralization of the stem, or malrotation of the stem within the femoral intramedullary canal produces an asymmetric distal cement mantle and may result in excessively thinned areas of cement, increased cement strains, and, in some cases, prosthesis on bone contact.^5,6,17-19,26 When compared to femoral components implanted in a neutral or a slight valgus position, those placed in varus malalignment (>5°) have shown an increased risk of cement fracture, radiolucencies, and loosening at both the stem-cement and cement-bone interfaces.¹⁹ Third-generation cement techniques have incorporated the use of proximal and distal femoral stem centralizers to improve alignment and obtain a more symmetric and uniform cement mantle.^7,18 The use of proximal and distal centralizers has resulted in significantly improved centralization, greater maintenance of optimal neutral alignment, and an increased occurrence of complete cement mantles >2 mm when compared to systems that did not use centralizers.^3,7,9,14,17 The use of distal centralizers alone improves centralization at the mid- and distal stem on the anteroposterior and lateral radiographs respectively and decreases the risk of deficient cement mantles (<1-2 mm) around the distal stem tip only in the anteroposterior plane.^7,9,17,18,23 In addition, four-finned centralizers have been shown to improve clinical alignment over three-finned centralizers.²⁴ It is unfortunate, however, that reports of distal centralizer fractures occurring during implantation have led to severe malalignment of the femoral component.^3,7,9,23

Defects or voids in the cement mantle affect the homogeneity of the cement mantle, the thickness of the cement, and the cement stresses experienced in the area of imperfection.^13,20 Although peripheral defects are typically considered detrimental,^{4,12,13,27,20} controversy exists over the long-term clinical effects of voids in the mid-substance of the cement mantle.^20,27,28

Clinical studies have shown that large voids, up to 5 mm in diameter, are often detrimental,^4,20,27,28 whereas smaller voids (<0.5 mm) are considered less significant.^16,28 The location of the imperfection has been shown to have the most relevance.^{9,12,16,18,20,27} Small voids in areas of the cement mantle known to experience high strains may result in premature fixation failure.^16,18,20

Insertion of the femoral component, with or without stem centralizers, can potentially create cement voids at the stem tip.^6,9,18 This phenomenon occurs as air is forced onto the cement surface and drawn into the distal cement mantle during initial insertion across the fluid boundary.^7,9,13

As the cement cures, heat generated from the exothermic reaction may further expand the trapped air, enlarging the void.^7,13,20 Coating the distal stem tip (and centralizer if present) with the same unit of cement used to fill the femoral canal, before stem insertion into the cement column, can reduce the incidence of distal voids.¹⁸ The development of a more streamlined centralizer has also been recommended to reduce the incidence of distal voids.⁹ Meticulous attention to cement preparation via centrifugation or mixing in a vacuum environment, drying the exposed bone and stem surface before stem insertion, use of pressure injection techniques, and avoiding toggling or rotation of the stem during curing of the cement will also reduce the incidence of defects within the cement mantle.^9,27

Variations in stem design and geometry affect stresses experienced in the cement mantle. Finite element analysis has demonstrated that larger diameter stems increase cement stresses from proximal to distal,^15,20 increasing detrimental cement strains at the femoral stem tip,^6,22 but decreasing proximal-medial mantle stresses when compared to thinner stem diameters.²⁹ Incorporating smaller diameter stems to effectively thicken cement mantles has been associated with more evenly distributed and decreased cement mantle stresses along the length of the femoral stem.^15,17,20 Finite element analysis has also demonstrated that increases in stem modulus of elasticity results in elevated cement mantle stresses.^6,15,20-22

Femoral components with sharp edges incorporated into the distal stem geometry or nontapered stems have been shown to increase distal cement mantle tip stresses.^4-6,17 In addition to decreasing stresses in the distal cement mantle,⁵ tapering of the femoral stem has been shown to reduce the introduction of air (voids) into the cement column during implant insertion.⁹

It has also been suggested^17,18,23 and shown⁵ that use of an “anatomic” femoral component, conforming to the bow typically present in the native proximal femur, may provide both a more uniform cement mantle and improved centralization of the stem than with use of a straight stem design, particularly when a distal centralizer is not used. The use of a highly polished stem may also be beneficial in reducing the occurrence of debonding at the cement-bone interface, decreasing cement stresses and improving the survival of cemented femoral components.^22,31,32

Stem subsidence, without the occurrence of cement fracture or loosening, may be possible when using highly polished and tapered stems due to creep that occurs in the cement allowing limited remodeling of the cement mantle.^31,32 Subsidence may actually optimize cement fixation and load distributions, and has been shown in finite element analyses to decrease cement mantle stresses.^31,32

The creation of debris initiated by cement fractures, debonding, and micromotion at the prosthesis-cement interface may be minimized by the use of highly polished stems that have a lower coefficient of friction when compared to rougher stem surfaces that may have more of a “sandpaper” effect.^22,32 Finally, surgeon knowledge of the broach vs. implant size in individual systems is mandatory to determine the space available for the cement mantle.¹⁷

The goal of cemented femoral components in THA is to achieve an adequately thick, symmetric, and homogeneous cement mantle around an appropriately sized and well-designed implant. These goals will effectively decrease detrimental stresses experienced in the cement mantle and limit the occurrence of cement fracture, debonding, osteolysis, and eventual aseptic loosening of the cemented femoral component.

Meticulous preparation of the femoral canal and proper cement preparation, insertion, and pressurization techniques are important in achieving a quality cement mantle. Use of smaller diameter, highly polished tapered stems with a lower modulus of elasticity and lack of sharp edges throughout its length will decrease detrimental stresses experienced in the cement mantle.

Lastly, knowledge of the implant system, adherence to recommended implantation techniques when using proximal and distal centralizers, and patience during insertion and curing of the cement are imperative to achieve an adequately thick, symmetric, and homogeneous cement mantle.

References

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Authors

From *the Department of Biomedical Engineering, University of Tennessee, Knoxville, Tenn, Oak Ridge National Laboratory/University of Tennessee Center for Musculoskeletal Research, Knoxville, Tenn, and Rocky Mountain Musculoskeletal Research Laboratory, Denver, Colo; and †the Orthopaedic Group, New Haven, Conn.