Effects of screw eccentricity on the initial stability of the acetabular cup in artificial foam bone of different qualities

Defining the canal for ischial and pubic screws in cup revision surgery

Purpose

When revising acetabular cups, it is often necessary to provide additional stabilisation with screws. In extensive defect situations, the placement of screws caudally in the ischium and/or pubis is biomechanically advantageous. Especially after multiple revision operations, the surgeon is confronted with a reduced bone stock and unclear or altered anatomy. In addition, screw placement caudally is associated with greater risk. Therefore, the present study aims to identify and define safe zones for the placement of caudal acetabular screws.

Methods

Forty-three complete CT datasets were used for the evaluation. Sixty-three distinctive 3D points representing bone landmark of interests were defined. The coordinates of these points were then used to calculate all the parameters. For simplified visualisation and intra-operative reproducibility, an analogue clock was used, with 12 o’clock indicating cranial and 6 o’clock caudal.

Results

A consistent accumulation was found at around 4.5 ± 0.3 hours for the ischium and 7.9 ± 0.3 hours for the pubic bone.

Conclusions

The anatomy of the ischium and pubis is sufficiently constant to allow the positioning of screws in a standardised way. The interindividual variation is low — regardless of gender — so that the values determined can be used to position screws safely in the ischium and pubis. The values determined can provide the surgeon with additional orientation intra-operatively when placing caudal acetabular screws.

Effect of different motor tasks on hip cup primary stability and on the strains in the periacetabular bone: An in vitro study

BACKGROUND

Excessive prosthesis/bone motions and the bone strains around the acetabulum may prevent osteointegration and lead to cup loosening. These two factors depend on post-operative joint loading. We investigated how Walking (which is often simulated) and Standing-Up from seated (possibly more critical) influence the cup primary stability and periacetabular strains.

METHODS

Twelve composite hemipelvises were used in two test campaigns. Simplified loading conditions were adopted to simulate Walking and Standing-Up. For each motor task, a single-direction force was applied in load packages of increasing amplitude. Stable and unstable uncemented cups were implanted. Digital image correlation was used to measure implant/bone motions (three-dimensional translations and rotations, both permanent and inducible), and the strain distribution around the acetabulum.

FINDINGS

When stable implants were tested, higher permanent cranial translations were found during Walking (however the resultant migrations were comparable with Standing-Up); higher rotations were found for Standing-Up. When unstable implants were tested, motions were 1-2 order of magnitude higher. Strains increased significantly from stable to unstable implants. The peak strains were in the superior aspect of the acetabulum during Walking and in the superior-posterior aspect of the acetabulum and at the bottom of the posterior column during Standing-Up.

INTERPRETATION

Different cup migration trends were caused by simulated Walking and Standing-Up, both similar to those observed clinically. The cup mobilization pattern depended on the different simulated motor tasks. Pre-clinical testing of new uncemented cups could include simulation of both motor tasks. Our study could also translate to indication of what tasks should be avoided.

Possible Vascular Injury Due to Screw Eccentricity in Minimally Invasive Total Hip Arthroplasty

BACKGROUND

Vascular injury during minimally invasive total hip arthroplasty (THA) is uncommon, yet a well-recognized and serious issue. It emerges because of non-visibility of vascular structures proximal to the pelvic bone during reaming, drilling holes, and fixing of screws. Numerous studies have found that screw fixation during cementless THA is beneficial for the initial stability of cup; yet, no anatomical guidelines support angular eccentric screw fixation.

MATERIALS AND METHODS

In this study, we obtained the pelvic arterial-phase computed tomographic data of thirty eight humans and reconstructed the three-dimensional models of osseous and vessel structures. We performed the surgical simulation to fix these structures with cementless cups and screws with angular eccentricities.

RESULTS

The effect of screw eccentricities (angular eccentricities of ±17° and ±34°) on the vascular injury was determined. Measurement between screw and adjoining vessels was performed and analyzed statistically to ascertain a comparative risk study for blood vessels that are not visible during surgery.

CONCLUSION

Authors similarly discussed the significant absence of appreciation of quadrant systems proposed by Wasielewski et al. on eccentric screws. Adjustment of quadrant systems provided by Wasielewski et al. is required for acetabular implants with eccentric holes for fixation of acetabular screws.

Evaluation of the effect of custom burr holes on a surgeon's sense of screw fixation in revision porous metal cups

BACKGROUND/AIMS

It is common practice to burr custom holes in revision porous metal cups for screw insertion. The objective of this study was to determine how different hole types affect a surgeon's sense of screw fixation.

METHODS

Porous revision cups were prepared with pre-drilled and custom burred holes. Cups were held in place adjacent to synthetic bone material of varying density. Surgeons inserted screws through the different holes and materials. Surgeon subjective rating, compression, and torque was recorded.

RESULTS

The torque achieved was greater (p = 0.002) for screws through custom holes than pre-fabricated holes in low and medium density material, with no difference for high density. Peak compression was greater (p = 0.026) through the pre-fabricated holes only in high density material.

CONCLUSION

Use of burred holes affects the torque generated, and may decrease the amount of cup-acetabulum compression achieved.

The effect of dorsal rim loss on the initial stability of the BioMedtrix cementless acetabular cup

Background

Loss of dorsal acetabular rim (DAR) is a common sequela to canine hip dysplasia. The purpose of this study is to evaluate the effect of DAR loss on the initial stability of the cementless (BFX) acetabular cup. BFX cups were implanted into foam blocks reamed to resemble acetabulae with simulated 0, 25, 50, and 75% DAR loss. Models were tested in edge loading of the lateral surface of the cup with an indenter, and in centered loading with an articulated femoral prosthesis. Additionally, cups were implanted into paired cadaveric canine hemipelves with either no DAR depletion, or removal of 50% of the DAR, and acutely loaded to failure with an articulated femoral prosthesis.

Results

Mean load measured at 1 mm cup displacement during edge loading was not significantly different in foam blocks with loss of 0, 25, 50, and 75% DAR (360 ± 124 N, 352 ± 42 N, 330 ± 81 N, 288 ± 43 N, respectively; P = 0.425). Mean load to failure with centered loads was greatest in blocks with 0% DAR loss (2828 ± 208 N; P < 0.001), but was not significantly different between 25, 50, and 75% DAR loss (2270 ± 301 N, 1924 ± 157 N, 1745 ± 118 N). In cadaveric testing, neither mean load to failure (P = 0.067), stiffness (P = 0.707), nor energy (P = 0.228) were significantly different in control hemipelves and those with 50% depletion of the DAR. Failure in all acetabulae occurred due to acetabular bone fracture at forces in supraphysiologic ranges.

Conclusions

BFX cup stability under normal physiologic loads does not appear to be compromised in acetabulae with up to 50% DAR loss.

Locking versus standard screw fixation for acetabular cups: is there a difference?

INTRODUCTION

Total hip replacement has been established as a valid treatment option for displaced subcapital fractures. However, insufficient primary fixation may be the reason for early loosening in these osteoporotic patients. Primary fixation of the cup is usually achieved by press-fit fixation that can be enhanced using screws. Locking the screws into their respective cups may seem to improve the primary fixation of the construct, as locked plates proved superior fixation for osteoporotic fractures.

METHODS

The study consisted of three groups: in each group, three cups were fixed into blocks of foam bone using press-fit technique. In the first group, no additional screws were used, in the second group two standard screws were inserted, while in the third group two acetabular screws were cemented into the cup to simulate locked screw fixation. Load was applied onto the rim of the acetabular component to cause shearing between the cup and the block. Cup fixation was examined by a loading machine that acquired load versus displacement. The stiffness (load vs. displacement) was calculated.

RESULTS

Screws, either locked or non-locked, enhanced cup fixation by 26 % (p value <0.01). No significant changes were found between the locking and non-locking screws groups.

DISCUSSION

These experimental results indicate that acetabular screws enhance primary cup fixation. This may become significant in conditions where the acetabular bone stock is suboptimal, such as when performing total hip arthroplasty after displaced subcapital fractures. However, there is no superiority for locked screws over standard screw fixation.

A cementless, elastic press-fit socket with and without screws

BACKGROUND

The acetabular component has remained the weakest link in hip arthroplasty regarding achievement of long-term survival. Primary fixation is a prerequisite for long-term performance. For this reason, we investigated the stability of a unique cementless titanium-coated elastic monoblock socket and the influence of supplementary screw fixation.

PATIENT AND METHODS

During 2006-2008, we performed a randomized controlled trial on 37 patients (mean age 63 years (SD 7), 22 females) in whom we implanted a cementless press-fit socket. The socket was implanted with additional screw fixation (group A, n = 19) and without additional screw fixation (group B, n = 18). Using radiostereometric analysis with a 2-year follow-up, we determined the stability of the socket. Clinically relevant migration was defined as > 1 mm translation and > 2º rotation. Clinical scores were determined.

RESULTS

The sockets without screw fixation showed a statistically significantly higher proximal translation compared to the socket with additional screw fixation. However, this higher migration was below the clinically relevant threshold. The numbers of migratory sockets were not significantly different between groups. After the 2-year follow-up, there were no clinically relevant differences between groups A and B regarding the clinical scores. 1 patient dropped out of the study. In the others, no sockets were revised.

INTERPRETATION

We found that additional screw fixation is not necessary to achieve stability of the cementless press-fit elastic RM socket. We saw no postoperative benefit or clinical effect of additional screw fixation.

Experimental Evidence of Impingement Induced Strains at the Interface and the Periphery of an Embedded Acetabular Cup Implant

After total hip arthroplasty, impingement of implant components may occur during every-day patient activities causing increased shear stresses at the acetabular implant–bone interface. In the literature, impingement related lever-out moments were noted for a number of acetabular components. But there is little information about pelvic load transfer. The aim of the current study was to measure the three-dimensional strain distribution at the macrostructured hemispherical interface and in the periphery of a standard acetabular press-fit cup in an experimental implant-bone substitute model. An experimental setup was developed to simulate impingement loading via a lever arm representing the femoral component and the lower limb. In one experimental setup 12 strain gauges were embedded at predefined positions in the periphery of the acetabular cup implant inside a tray, using polyurethane composite resin as a bone substitute material. By incremental rotation of the implant tray in steps of 10 and 30 deg, respectively, the strains were measured at evenly distributed positions. With the described method 288 genuine strain values were measured in the periphery of an embedded acetabular cup implant in one experimental setup. In two additional setups the strains were evaluated at different distances from the implant interface. Both in radial and meridional interface directions strain magnitudes reach their peak near the rim of the cup below the impingement site. Values of equatorial strains vary near zero and reach their peaks near the rim of the cup on either side and in some distance from the impingement site. Interestingly, the maximum of averaged radial strains does not occur, as expected, close to the interface but at an interface offset of 5.6 mm. With the described experimental setup it is now possible to measure and display the three-dimensional strain distribution in the interface and the periphery of an embedded acetabular cup implant. The current study provides the first experimental proof of the high local stresses gradients in the direct vicinity of the impingement site. The results of the current study help for a better understanding of the impingement mechanism and its impact on acetabular cup stability.

Artificial organs 2010: a year in review

In this Editor's Review, articles published in 2010 are organized by category and briefly summarized. As the official journal of The International Federation for Artificial Organs, The International Faculty for Artificial Organs, and the International Society for Rotary Blood Pumps, Artificial Organs continues in the original mission of its founders "to foster communications in the field of artificial organs on an international level."Artificial Organs continues to publish developments and clinical applications of artificial organ technologies in this broad and expanding field of organ Replacement, Recovery, and Regeneration from all over the world. We take this time also to express our gratitude to our authors for offering their work to this journal. We offer our very special thanks to our reviewers who give so generously of time and expertise to review, critique, and especially provide such meaningful suggestions to the author's work whether eventually accepted or rejected and especially to those whose native tongue is not English. Without these excellent and dedicated reviewers the quality expected from such a journal could not be possible. We also express our special thanks to our Publisher, Wiley-Blackwell, for their expert attention and support in the production and marketing of Artificial Organs. In this Editor's Review, that historically has been widely received by our readership, we aim to provide a brief reflection of the currently available worldwide knowledge that is intended to advance and better human life while providing insight for continued application of technologies and methods of organ Replacement, Recovery, and Regeneration. We look forward to recording further advances in the coming years.