341Bulletin of the NYU Hospital for Joint Diseases 2009;67(4):341-6

Kummer FJ, Pereira G, Schachter AK, Jaffe WJ. Femoral component positioning in resurfacing arthroplasty: effects on cortical strains. Bull NYU Hosp Jt Dis. 2009;67(4):341-6.

Abstract

Previous studies have suggested that femoral component positioning in resurfacing arthroplasty may affect strains in the femoral neck that could lead to decreased implant longevity, A strain gaged, Sawbones® model was used to de-termine the femoral neck strains for a variety of resurfacing head translations and angulations. We found that head posi-tions affected strain distributions, most positions leading to increased neck strains, often over 100%, with the exception being a varus head position where the superior neck strains decreased over 50%. Although the clinical meaning of these findings is unclear, it could be of concern for stress-shielding or fatigue fracture of the femoral neck.

Surface replacement arthroplasty (SRA) has regained popularity as an alternative to total hip arthroplasty for the treatment of degenerative disease of the hip

in younger patients, due to the use of modern metal-on-metal bearings and improved designs. Despite purported advantages of bone preservation, anatomic restoration, and physiological loading of the proximal femur, hip resurfacing can increase the risk for femoral neck fracture that has been reported as between 0.83% and 1.46%.1,2 A recent review of previous retrospective studies noted a relationship between

femoral neck fracture and varus placement of the femoral component, intraoperative notching of the neck, older ages, and female gender.3 Additionally, fracture may be related to progressive osteonecrosis or stress shielding. Laboratory studies4-14 have used mathematical modeling and strain gage instrumented femurs to demonstrate changes in femoral neck strains after implantation of a femoral component but have generally been limited to one or two component positions. To evaluate the effects of femoral component positioning, we measured the changes in strains in the femoral neck with nine component positions in relation to the anatomical neck axis: correct (coaxial with the femoral neck), 10° of varus, valgus, anteversion, and retroversion and 5 mm of translation superiorly, inferiorly, anteriorly, and posteriorly. Sawbones® third generation composite femurs (Pacific Research Labo-ratories, Inc., Vashon, Washington) were used to provide a reproducible model for testing.

Materials and MethodsThe experiment was conducted in several steps: 1. deter-mination of optimal strain gage placement; 2. comparison of femoral neck strains in intact Sawbones® and cadaver femurs; and 3. determination of femoral neck cortical strain changes for various femoral component placements in Sawbones® femurs.

Experimental ProtocolStrain GagingThis experiment used strain gages to measure outer “cor-tex” strains in Sawbones® femurs for a variety of surface replacement head positions. We sought to mount the gages close to the surface replacement heads in a similar, reproducible position for all femurs. As gages could not be mounted exactly (same positions and orientations) and strain gradients in the neck are not known, each specimen served as its own control with measurements taken before

Femoral Component Positioning in Resurfacing ArthroplastyEffects on Cortical Strains

Frederick J. Kummer, Ph.D., Gavin Pereira, M.D., Aaron K. Schachter, M.D., and William L. Jaffe, M.D.

Frederick J. Kummer Ph.D., is Professor, NYU School of Medi-cine, and Associate Director, Musculoskeletal Research Center, Department of Orthopaedic Surgery. Gavin Pereira, M.D., is a Fellow and Aaron K. Schachter, M.D., a research assistant within the Department of Orthopaedic Surgery. William L. Jaffe, M.D., is Clinical Professor, NYU School of Medicine, within the Division of Adult Reconstructive Surgery, and Vice Chairman, Department of Orthopaedic Surgery, NYU Hospital for Joint Diseases, NYU Langone Medical Center, New York, New York.Correspondence: Frederick Kummer, Ph.D., 301 E 17th Street, NYU Hospital for Joint Diseases, New York, New York 10003; fkummer@yahoo.com.

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and after surface replacement head mounting, Pre-wired simple and Rosette strain gauges (KFG-3-120-C1-11L1M2R and KFG-3-120-D17-11L1M2S, Omega, Stamford, Connecticut) were used. Rosette gages consist of three linear gages that are stacked at a fixed angle between the gages. A single rosette gauge was ap-plied to the superior (lateral) femoral neck, such that it was placed 5 mm from the closest position to where the surface replacement femoral head would be located in the valgus position as measured from a radiograph template. The other three linear strain gauges were applied to the anterior, posterior, and inferior aspects of the femoral neck using similar templating to locate the closest respective, resultant replacement head positions. The axes of all gages were aligned with that of the femoral neck. They were all applied with cyanoacrylic adhesive after cleaning and degreasing of the neck with acetone and coated with a thin layer of silicone for protection. The gages were attached to a multiplexed data acquisition system (Microlink 770, Biodata Ltd., Manchester, United Kingdom), which al-lowed for simultaneous balancing of the strain gage bridges (precision resistors used for completion) and sample rates to 50 Hz. Data readings were started after each specimen was given two minutes to equilibrate and taken for a min-ute, which resulted in approximately 500 separate readings from each gage.

Specimen Preparation, Mounting, and LoadingThe femurs were cut transversely 20 cm below the tip of the grater trochanter and potted with acrylic cement into 5x5x5 cm square aluminum tubes using a holding fixture to insure position and verticality. These specimens were held in an angle vise attached to a low friction X-Y table attached to

the testing machine actuator. This support was chosen on the basis of a previous study of femoral strains produced by various support conditions.15 A trochanteric or ilio-tibial (IT) band force was not used. Metal surface replacement acetabular cups were mounted with acrylic cement in a metal holder at 45° inclination and attached to the load cell of the testing machine at 15° anteversion to the necks of the femur specimens and used for loading of surface replacement heads. For loading of nonsurface replacement heads, a 3-mm thick sheet of silicone rubber was placed between the heads and an aluminum disc that was supported by a ball bearing slide mount. An MTS (MTS Systems. Eden Prarrie, Minnesota) was used to load the specimens to 1400N (load control feedback). Figure 1 shows this experimental set-up.

Comparison of Strains with Intact Sawbones® and Cadaver FemursThree proximal cadaveric femurs and three Sawbones® fe-murs were mounted and strain gauges applied as described above. The cadaver femurs were planed in the area of gage attachment with a knife blade to create a smooth surface and dehydrated and degreased with acetone and chloroform before application of the cyanoacrylate adhesive. They were then oriented at 15° of adduction (approximate coronal position of hip at normal heel strike) and loaded to 1400 N. A great variability in strains between the human and Sawbones® femurs was observed. This was attributed to the variation in the neck-shaft angles of the human femurs. In order to eliminate this, the cadaver femurs were reoriented in the vise so that their necks replicated the same angle as the Sawbones® (approximately140°) and re-tested.

Loading of Surface Replacement Heads Twelve (including three spares) Sawbones® proximal femurs were mounted and strain gauges attached in the same manner as described above. Each intact femur was loaded to 1400N and the strains recorded for each specimen. Measurements were done with the femoral shafts vertical, at 15° adduction

Figure 1 Experimental set-up showing an intact Sawbones® femur being tested in the vertical orientation.

Figure 2 Cormet surface replacement implant showing the femoral head and acetabulum. The stem of this implant facilitates head positioning in the femoral neck.

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and at 15° abduction to determine initial strains as a control. Each femur was then prepared to accept femoral com-ponent heads (52 mm Cormet, Corin, Cirencester, United Kingdom) (Fig. 2) that were to be oriented in one of nine positions in relation to the anatomical neck axis: correct (coaxial with the femoral neck), 10° of varus, valgus, an-teversion, retroversion, and 5 mm of translation superiorly, inferiorly, anteriorly, and posteriorly. For the correctly positioned head, an initial guide wire was introduced along a neutral axis (longitudinal center of the femoral neck), using the caliper-like jig designed for this purpose. The position of the guide wire was confirmed on radiographs (Fig. 3). For the other specimens, attachments were added to the jig for control of angulation or translation of the guide wire. The positions of these guide-wires were checked on orthogonal radiographs (Fig. 3). The Sawbones® femoral heads were then reamed with a cylindrical reamer, planed, and the chamfers cut, using the Cormet instruments. Care was taken not to notch the neck in any direction nor damage the already mounted strain gauges. This was avoided by using the sizing guide as a measure-ment template during reaming. Also, a rubber barrier was used to protect the strain gauges. The cylindrical reamer was cooled in a bucket of ice at regular intervals to prevent

thermal damage to the strain gauges. The femoral components were then cemented on the prepared femurs with low-viscosity acrylic bone cement. Additional orthogonal radiographs were taken to verify head position. The specimens were loaded to 1400 N and strain measure-ments were recorded for each of the three femoral angula-tions. Each test was repeated three times. One specimen had problems with the strain gages and was redone; additional correct and varus head placement specimens were prepared and tested to evaluate test reproducibility.

Data AnalysisStrain measurements were recorded as microstrain and an average value for each gage determined. The three readings from the rosette gages were converted to the principle strain and direction using standard gage equations. The changes in strain were expressed as a percentage increase/decrease from the corresponding, non-implanted femoral neck strain values.

ResultsTable 1 shows that the inferior and superior (maximum principle) femoral neck strains were, in general, similar for

Figure 3 Left, Attachment of location jig to Sawbones® femur. Middle, Location of center pin. Right, Locating of 5 mm inferior off-set guide pin.

Table 1 Comparison of Microstrains for Cadaver and Sawbones® Femurs (Sawbones® at 15° Adduction, Cadaver Adductions Adjusted for Parallel Neck Orientation)

Specimen Maximum Inferior Anterior Posterior Superior Strain Strain Strain StrainCadaver 1000 -5500 500 300 1100 -4000 450 1000 3000* -8000* 650* 1220*Sawbones® 1100 -4050 -50 -880 1050 -3900 10 -920 950 -4200 -30 -790*Femur was osteoporotic on radiograph.

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the cadaver and Sawbones® femurs, except for one cadaver femur that appeared osteoporotic on radiographs. However, the anterior and posterior strains were compressive for the Sawbones® and tensile for the cadaver femurs. All of the surface replacement heads were within 20% of their intended experimental positions as determined by orthogonal radiograph measurements (Table 2). The principle superior strains for the femoral neck (in 15° adduction) were 25% to 80% greater for all angulations of the surface replacement head except for varus positioning where it was approximately 50% less (Fig. 4). Although there were variations in the directions of the principle strains, their meaning is unclear and could be due to shifting of the specimen on the X-Y table. The principle superior strains from the femoral neck (in 15° adduction) were 35% to 120% greater for all transla-tions of the surface replacement head, except for inferior

positioning where it was approximately 50% less (Fig. 5). The other strain readings from the femoral neck (in 15° adduction) were less or similar for all angulations of the surface replacement head, except for the posterior gage read-ing with varus head positioning where it was approximately 300% greater (Fig. 6). The remaining strain readings from the femoral neck (in 15° adduction) were affected by all translations of the surface replacement head. The posterior gage reading with varus head positioning increased approximately 100% in inferior and posterior translation and decreased approximately 225% in anterior translation (Fig. 7). The angle of load application (femoral orientation) also affected the strain magnitudes. As seen in Figure 8, the principle superior strains for three surface replacement head positions changed when the femur was in adduction or abduction. The strains for the varus head position were

Table 2 Actual Specimen Head Positions (Neutral is Centered Within the Neck and Parallel to its Longitudinal Axis)

Axis of Axis of Orientation on Actual Orientation on ActualSpecimen AP Radiograph Value Lateral Radiograph Value

Correct-Neutral Neutral 1 mm Neutral 0 mmValgus Valgus 8° Neutral 1 mmVarus Varus 11° Neutral 0 mmSuperior Translation Translated superiorly 6 mm Neutral 0 mm (parallel to neutral) Inferior Translation Translated inferiorly 5 mm Neutral 0 mm (parallel to neutral) Anteversion Neutral 11° Anteverted 1 mmRetroversion Neutral 9° Retroverted 1 mmAnterior Translation Neutral 4 mm Translated anteriorly 1 mm (parallel to neutral)Posterior Translation Neutral 6 mm Translated anteriorly 0 mm (parallel to neutral)

Figure 4 Principle superior strains and directions for an-gulations of the resurfacing head. Axis values are percent change from an intact Sawbones® femur at 15° adduction; angles are change from 0.0°. *Actual valgus value is -50%.

Figure 5 Principle superior strains and directions for transla-tions of the resurfacing head. Axis values are percent change from an intact Sawbones® femur at 15° adduction; angles are change from 0.0°. *Actual inferior value is -50%.

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50% lower in both positions and, generally, 100% higher for the correct and valgus head positions. The two repeated tests for the surface replacement heads in varus and the correct orientation had strains that were within 20% of the original tests.

DiscussionWe found the varus positioning of the surface replacement head resulted in decreased superior strain in the femoral neck, as has been seen by several previous investigations.4,9 However, a recent study by Vail and colleagues demon-strated a slight increase.16 Although this head position results in lateral translation of the load and an decreased lever arm of the neck, this does not account for changes of such magnitude. What is probably occurring is that the longitudinal, tensile loads in the neck are being partially transformed to vertical shear stresses. This transformation seems to be supported by the observation that the compres-sive loads and resultant strains on the inferior neck do not appreciably vary with respect to head position, as would be expected if the neck was solely acting as a beam in

bending. Other angular head positions generally resulted in an increased superior strain, as has been seen by other studies.5,10,13

Anterior or posterior head translations resulted in ex-pected changes in the anterior and posterior gages. Inferior head translation also affected these two gages; superior translation had little effect. It should be noted that these are linear gages and are only partially affected by shear strains. Furthermore, these two locations showed a major difference between the strains seen in Sawbones® and cadaver femurs. There are several limitations to this experiment, one being that only single specimens were tested for each head position. We used Sawbones® femurs to provide specimen uniformity and tried to standardize gage positions. We did repeat fabrication and testing of two specimens (correct and varus) and found good agreement in strain data. We do not know the effect of gage position as only one location close to the implant was studied. Some studies7,9 suggest that there is a high strain gradient close to the im-plant; gage positions further down the neck might not have shown the same magnitude of changes. It is also important to know that only surface strains were measured; internal strains are unknown. We used simple loading and did not account for IT band effects and abductor loading. Although Sawbones® composite femurs are designed to replicate the mechanical properties of actual femurs, we did find some major differences in strains between the two and also between actual femurs. The Sawbones do not duplicate the internal stress transfer found in actual bone, as they lack the internal load distribution trabecular architecture. It is unclear what are the clinical implications of the increased or decreased strains in the femoral neck found in this study. The strains found in the cadaver femurs are within those shown to cause fatigue of cortical bone in the laboratory,17 and if the increased strains for certain head positions found with the Sawbones® femurs are effected

Figure 6 Anterior, posterior, and inferior strains for angula-tions of the resurfacing head. Values are percent change from an intact Sawbones® femur at 15° adduction.

Figure 7 Anterior, posterior, and inferior strains for transla-tions of the resurfacing head. Values are percent change from an intact Sawbones® femur at 15° adduction.

Figure 8 Effect of Sawbones® femur test angle on the percent change in the maximum superior neck strain for three surface replacement head positions.

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in the cadaver femurs, this could be of concern. However, bone remodeling could also modify this stress/strain state over time.

Disclosure StatementThis investigative report was supported by a grant from Stryker Orthopaedics, Mahwah, New Jersey.

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