Length of clinically proven cemented hip stems: State of the art or subject to improvement?

Cemented short-stem total hip arthroplasty: Characteristics of line-to-line versus undersized cementing techniques using a validated CT-based finite element analysis

Short stems are becoming increasingly popular in total hip arthroplasty as they preserve the bone stock and simplify the implantation process. Short stems are advised mainly for patients with good bone stock. The clinical use of short stems could be enlarged to patients with poor bone stock if a cemented alternative would be available. Therefore, this study aimed to quantify the mechanical performance of a cemented short stem and to compare the "undersized" cementing strategy (stem one size smaller than the rasp) with the "line-to-line" technique (stem and rasp with identical size). A prototype cemented short stem was implanted in eight pairs of human cadaveric femora using the two cementing strategies. Four pairs were experimentally tested in a single-legged stance condition; stiffness, strength, and bone surface displacements were measured. Subject-specific nonlinear finite element models of all the implanted femora were developed, validated against the experimental data, and used to evaluate the behavior of cemented short stems under physiological loading conditions resembling level walking. The two cementing techniques resulted in nonsignificant differences in stiffness and strength. Strength and stiffness as calculated from finite element were 8.7 ± 16% and 9.9 ± 15.0% higher than experimentally measured. Displacements as calculated from finite element analyses corresponded strongly (R ²  ≥ .97) with those measured by digital image correlation. Stresses during level walking were far below the fatigue limit for bone and bone cement. The present study suggests that cemented short stems are a promising solution in osteoporotic bone, and that the line-to-line and undersized cementing techniques provide similar outcomes.

Short vs Standard-Length Femoral Stems Cemented According to the "French Paradox": A Matched Paired Prospective Study Using Ein Bild Roentgen Analyze Femoral Component at Two-Year Follow-Up

BACKGROUND

The purpose of this prospective matched paired study is to compare the in vivo migration patterns using Ein Bild Roentgen Analyze femoral component of shortened vs standard-length stems cemented line-to-line in primary total hip arthroplasty (THA) at 2-year follow-up.

METHODS

We prospectively included the first 50 consecutive primary cemented THAs in 50 patients using a 12% shortened stem (AmisK group) of which design was derived from the original Charnley-Kerboull (CK) femoral components. These 50 patients were matched paired to 50 patients from a historical series of patients who underwent primary THAs using standard-length CK stems (CK group) with available long-term results, including Ein Bild Roentgen Analyze femoral component data at 2-year follow-up.

RESULTS

Body mass index was significantly higher (P = .007) in the AmisK vs the CK group. At the 2-year follow-up, the mean subsidence was 0.65 mm (0-1.40) in the AmisK group vs 0.68 mm (0.07-1.43) in the CK group (P = .73). When using a 1.5-mm threshold, none of the stems in either group was considered to have subsided. Femoral cortical thickening in zones 3 and 5 occurred in 6 of the 50 hips (12%) in the AmisK group vs 20 (40%) of the 50 hips in the CK group (P = .003).

CONCLUSION

Our study showed that a shortened highly polished double-tapered stem cemented line-to-line provided similar results including minimal subsidence as its standard-length counterpart, with significantly less distal femoral cortical thickening. However, longer term survival analysis still needs to be determined.

Press-fit reverse shoulder arthroplasty in case of advanced humeral bone loss - Is additional distal fixation necessary for primary stability?

BACKGROUND

Revision reverse shoulder arthroplasty (RSA) poses considerable surgical challenges. We hypothesized that a newly developed press-fit stem, which is modeled on the medullary canal of the supracondylar region of the distal humerus by a slight distal bend, achieve both correct fit and sufficient primary stability and that additional distal fixation by interlocking screws is favorable in case of advanced humeral bone loss.

METHODS

A modular tapered press-fit stem was implanted in 16 Sawbone humeri in three consecutively created defect situations (200 mm (experimental group type 3°), 160 mm (type 4°) and 120 mm (type 5°) bone length above the epicondylar line. In experimental groups type 4° and 5°, additional distal interlocking screw fixation with one to three screws was tested. Primary stability was investigated by measuring micromotions with a high-precision rotational setup.

FINDINGS

Highest relative micromotions were noted at the proximal end in experimental groups type 3° and type 4°, whereas in type 5° highest micromotions could be seen at the distal end. Overall micromotions were significantly lower in type 3° and increased with extended defect size. In experimental group type 5°, micromotions increased with reduced additional distal screw fixation.

INTERPRETATION

The examined press-fit stem did not provide sufficient primary rotational stability in all constructs without additional support. Advanced distal humeral bone loss had a strong impact on primary fixation. In experimental group type 5° with 120 mm bone remaining, it might be beneficial to use three distal interlocking screws in the supracondylar region in order to neutralize torque and to avoid early loosening.

How do the geometries of the broach handles relate to the distribution of force and moments in a femoral model?

The continuous improvement of minimally invasive hip endoprostheses surgery comes with a change in geometries of surgery instruments like the broach handles. Consequently, depending on the handles' curvature this results in a deviation between handle and femoral axis. Therefore, this study aimed to prove the influence of different handles' curvatures on the preparation of implant site and acting forces and moments in this process. Five femoral models attached to different handles (double-curved, single-curved, straight) were locked in a drop-weight device with standardize implantation forces and moments and five strokes were measured for each possible combination. Distribution of force and moment components was dependent on the handle's curvature, where the lowest variation from the standard force values was by the straight one (av:15.2% ± 0.5%) and the strongest discrepancies were exhibit by the double-curved one (av:54.3% ± 0.1%.). Moment values have also shown this trend with the lowest variation (12.4%-23.3%) by the straight one and the highest discrepancies (56,6%-90.9%) by the double-curved one. Results show that unguided axial impact introduces unwanted transverse forces and moments into the femur. Therefore, broach handles should be modified accordingly so that minimally invasive surgery remains feasible but unwanted forces or moments can still be compensated.

Comparison of Short Stems Versus Straight Hip Stems: A Biomechanical Analysis of the Primary Torsional Stability

Cementless straight stems show very good survival rates. However, the more distal force application of straight stems may lead to release-related proximal stress-shielding. Nevertheless, this technical brief had the objective of conducting a biomechanical in vitro analysis comparing short stems with established straight stems with respect to their primary torsional stability. Two cementless short hip stems and three cementless straight hip stems were implanted in n = 5 synthetic femora each. Torsional torques were applied into the hip stems at a continuous interval of ±7 Nm. Micromotions were measured by six inductive extensometers on four different measurement levels. At the proximal measuring point, significantly smaller relative micromotions of the CLS® prosthesis could be detected compared to all other stem models (p < 0.05). In all stem models, smallest relative micromotions were found at the metaphyseal/diaphyseal measuring point. Only at the measuring point of the distal tips of the straight stems, statistically significantly lower relative micromotion of the CLS® stem compared to the Trendhip® stem could be found (p < 0.01). All the investigated stems generally display a rather comparable anchoring pattern and an almost physiological force application. Since the comparatively long straight stems present an anchoring pattern nearly identical to that of the examined short stems, a shortening of the established straight stems could be taken into consideration. This would offer the advantage of minimally invasive surgery and bone-saving resection as well as preservation of cancellous bone in case a revision would become necessary.

In vitro examination of the primary stability of three press-fit acetabular cups under consideration of two different bearing couples

BACKROUND

For preclinical statements about the anchoring behavior of prostheses, the primary stability of the prosthesis is of special importance. It was the aim of this study to examine and compare the relevant relative micromotions of three different acetabulum prostheses by introducing three-dimensional torques.

METHODS

The cups were implanted under standard conditions into an anatomical artificial bone model. Three-dimensional torques were applied to the acetabular cups. Taking into account the resulting frictional moments of two different bearing couples, ceramic-on-ceramic and ceramic-on-polyethylene, the relative micromotions of the cups were recorded as maximum total micromotion, translational and rotational micromotion, and the primary stability values of the three cups were compared.

RESULTS

Relative micromotion of all cup models was always significantly smaller with the CoC bearing couples than with the CoP bearing couples (p < 0.001). The rotational micromotion was always lower (p < 0.001) than the translational micromotion, and the rotational as well as the translational micromotions were each always lower than the maximum total micromotion (p < 0.001, p < 0.010). The thinnest-walled cup system always showed the largest relative micromotions.

CONCLUSION

The results of our study can be interpreted as indicating that the low relative micromotions of all cups - irrespective of the use of CoC or CoP bearing couples - are within an acceptable range favoring secondary osseointegration of the implants. Furthermore, we were able to show that the cup wall thickness and the surface quality of the cup systems have an influence on the primary stability and the elastic deformability of the examined cup systems.

Effect of bearing friction torques on the primary stability of press-fit acetabular cups: A novel in vitro method

Aseptic loosening is the main reason for revision of total hip arthroplasty, and relative micromotions between cementless acetabular cups and bone play an important role regarding their comparatively high loosening rate. Therefore, the aim of the present study was to analyze the influence of resulting frictional torques on the primary stability of press-fit acetabular cups subjected to two different bearing partners. A cementless press-fit cup was implanted in bone-like foam. Primary stability of the cup was analyzed by determining spatial total, translational, and rotational interface micromotions by means of an eddy current sensor measuring system. Torque transmission into the cup was realized by three synchronous servomotors considering resultant friction torques based on constant friction for ceramic-on-ceramic (CoC: μ = 0.044; max. resultant torque: 1.5 Nm) and for ceramic-on-polyethylene (CoP: μ = 0.063; max. resultant torque: 1.9 Nm) bearing partners. Rotational micromotion of CoC was 8.99 ± 0.85 µm and of CoP 13.39 ± 1.43 µm. Translational micromotion of CoC was 29.93 ± 1.44 μm and of CoP 39.91 ± 2.25 μm. Maximum total relative micromotions were 37.10 ± 1.07 μm for CoC and 51.64 ± 2.18 μm for CoP. Micromotions resulting from CoC were statistically lower than those resulting from CoP (p < 0.05). The described 3D-measuring set-up offers a novel in vitro method of measuring primary stability of acetabular cups. We can therefore conclude, that primary stability of acetabular cup systems can be observed using either the lower friction curve (CoC) or the higher friction curve (CoP). In future studies different cup designs or cup fixation mechanisms may be tested and compared in vitro and assessed prior to implantation. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2745-2753, 2018.

Standard and line-to-line cementation of a polished short hip stem: Long-term in vitro implant stability

The current trend is toward shorter hip stems. While there is a general agreement on the need for a cement mantle thicker than 2 mm, some surgeons prefer line-to-line cementation, where the mantle has only the thickness provided by the cement-bone interdigitation. The aim of this study was to assess if a relatively short, polished hip stem designed for a standard cementation can also be cemented line-to-line without increasing the risk of long-term loosening. Composite femurs with specific open-cell foam to allow cement-bone interdigitation were used. A validated in-vitro biomechanical cyclic test replicating long-term physiological loading was applied to femurs where the same stem was implanted with the Standard-mantle (optimal stem size) and Line-to-line (same rasp, one-size larger stem). Implant-bone motions were measured during the test. Inducible micromotions never exceeded 10 μm for both implant types (differences statistically not-significant). Permanent migrations ranged 50-300 μm for both implant types (differences statistically not-significant). While in the standard-mantle specimens there was a pronounced trend toward stabilization, line-to-line had less tendency to stabilize. The cement cracks were observed after the test by means of dye penetrants: The line-to-line specimens included the same cracks of the standard-mantle (but in the line-to-line specimens they were longer), and some additional cracks. The micromotions and cement damage were consistent with those observed in-vitro and clinically for stable stems, confirming that none of the specimens became dramatically loose. However, it seems that for this relatively short polished stem, standard-mantle cementation is preferable, as it results in less micromotion and less cement cracking. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2736-2744, 2018.

Analysis of the elastic bending characteristics of cementless short hip stems considering the valgus alignment of the prosthetic stem

BACKGROUND

The resultant hip force causes a varus torque which must be compensated by a shear force couple depending on the stem alignment of the prosthesis. Since the prosthesis is substantially less flexible than the bone, the interior of the femur is stiffened over the entire prosthesis length. The present study thus aims at analyzing short-stem prostheses for its elastic bending characteristics, considering inappropriate valgus alignment of the prosthetic stem.

METHODS

Five short stem prostheses were implanted each in synthetic femora in a standardized manner - in neutral and valgus stem alignments. Bending movements were recorded applying a tilting torque M_X of ±3.5 Nm in medio-lateral direction. Variance analyses and Friedman tests were used. A P-value <.05 was considered statistically significant.

FINDINGS

Bending movements b₁-b₆ showed significant differences (P < .05). It could be shown that different stem alignments (P < .05) and different measuring points had a highly significant influence (P < .001) on the relative movements. Compared to the AIDA®, the MiniHip™ as well as the Metha® stiffened the femur to a higher degree (P < .001).

INTERPRETATION

Regarding the elastic bending behavior we see a relevant influence of the stems´ design. We conclude that the short-stem principle does not necessarily require the shortest possible prosthesis but rather a long and thin extending stem tip to optimize the lever ratios, ensuring a more physiological bending behavior of the femur. In addition, without sufficient anchoring of the prosthesis, the valgus stem alignment could favor tilting of the implant and should therefore be avoided.

Optimization of a Functionally Graded Material Stem in the Femoral Component of a Cemented Hip Arthroplasty: Influence of Dimensionality of FGM

The longevity of hip prostheses is contingent on the stability of the implant within the cavity of the femur bone. The cemented fixation was mostly adopted owing to offering the immediate stability from cement-stem and cement-bone bonding interfaces after implant surgery. Yet cement damage and stress shielding of the bone were proven to adversely affect the lifelong stability of the implant, especially among younger subjects who tend to have an active lifestyle. The geometry and material distribution of the implant can be optimized more efficiently with a three-dimensional realistic design of a functionally graded material (FGM). We report an efficient numerical technique for achieving this objective, for maximum performance stress shielding and the rate of early accumulation of cement damage were concurrently minimized. Results indicated less stress shielding and similar cement damage rates with a 2D-FGM implant compared to 1D-FGM and Titanium alloy implants.

Minimizing Stress Shielding and Cement Damage in Cemented Femoral Component of a Hip Prosthesis through Computational Design Optimization

The average life expectancy of many people undergoing total hip replacement (THR) exceeds twenty-five years and the demand for implants that increase the load-bearing capability of the bone without affecting the short- or long-term stability of the prosthesis is high. Mechanical failure owing to cement damage and stress shielding of the bone are the main factors affecting the long-term survival of cemented hip prostheses and implant design must realistically adjust to balance between these two conflicting effects. In the following analysis we introduce a novel methodology to achieve this objective, the numerical technique combines automatic and realistic modeling of the implant and embedding medium, and finite element analysis to assess the levels of stress shielding and cement damage and, finally, global optimization, using orthogonal arrays and probabilistic restarts, were used. Applications to implants, fabricated using a homogeneous material and a functionally graded material, were presented.

Results of total hip arthroplasty using a bionic hip stem

PURPOSE

The trabecular-orientated bionic hip stem was designed to mimic the natural force transmission through the femur in total hip arthroplasty, resulting in supposedly longer prosthesis survivability. The aim of this study was to compare the second-generation bionic hip stem to a standard uncemented hip stem.

METHODS

A group of 18 patients (21 hips) who underwent total hip arthroplasty with a bionic stem (bionic group) was compared with a historic group of 12 patients (12 hips) treated with standard anatomic hip stem (control group). During the first year after the procedure, the densitometric measurements of the bone around the prosthesis were taken. Radiographic and clinical assessments were additionally performed preoperatively and at the three month, six month, one year and three year follow-ups in the bionic group.

RESULTS

In the bionic group, one patient was revised for aseptic loosening and 16 patients (19 hips) were available to the final follow-up. A significant decrease of bone mineral density was found in Gruen zones 3, 4 and 5 in the bionic group, and in zone 7 in both groups. The bionic group had a significantly higher bone mineral density in Gruen zone 1 at the one year follow-up. At the final follow-up, all prostheses were radiologically stable in both groups.

CONCLUSIONS

Provided that a good implant position is achieved, comparable short-term results can be obtained using a bionic stem. Still, a decrease of bone mineral density in Gruen zone 7 occurred in both groups. Further studies are required to determine survivability of the bionic stem.