Bone stresses before and after insertion of two commercially available distal ulnar implants using finite element analysis

Finite element analysis of intraosseous distal radioulnar joint prosthesis

BACKGROUND

Joint replacement is one of the options to retrieve the interosseous distal radioulnar joint (DRUJ) function. DRUJ prosthesis has recently been introduced clinically to treat DRUJ instability. This article analyzes the biomechanical behavior of the prosthesis during different loadings by the finite element method.

METHODS

CT images of a healthy 33 years old man were used to construct the three-dimensional geometry of the forearm bone. Then two models, a healthy foreman (Model A) and a damaged model with an inserted interosseous prosthesis (Model B), were constructed to analyze and compare the foreman's biomechanical behavior under different loading conditions using the finite element method. Both models were examined during pronation and supination with 500, 1000, 2000, and 5000 N.mm values. Also, both models were subjected to volar and dorsal loads with values of 10, 30, and 50 N and traction force with 100, 150, and 200 N.

RESULTS

Maximum and minimum principal stresses were evaluated for bones in all conditions, and von Mises stress was considered for the prosthesis and fixing screws. In supination, the maximum stress in Model A is significantly higher than the Model B. However, the maximum principal stress of both models is similar during volar and dorsal loading. In Model A, the maximum principal stress in traction is much smaller than in Model B. The absolute value of minimum principal stress in pronation and supination in Model B is higher than in Model A. The prostheses and screws are subjected to higher stresses during pronation than supination. Also, the amount of stress created in prostheses and screws during volar and dorsal loading is almost equal. In traction loading, screws are subjected to significantly high stresses.

CONCLUSION

Our study indicates that the interosseous DRUJ prosthesis can perform the foreman's normal daily activities. This prosthesis provides the ability similar to a normal hand.

LEVEL OF EVIDENCE

IV.

Structural analysis of hollow versus solid-stemmed shoulder implants of proximal humeri with different bone qualities

Stress shielding of the proximal humerus following total shoulder arthroplasty (TSA) can promote unfavorable bone remodeling, especially for osteoporotic patients. The objective of this finite element (FE) study was to determine if a hollow, rather than solid, titanium stem can mitigate this effect for healthy, osteopenic, and osteoporotic bone. Using a population-based model of the humerus, representative average healthy, osteopenic, and osteoporotic humerus FE models were created. For each model, changes in bone and implant stresses following TSA were evaluated for different loading scenarios and compared between solid versus hollow-stemmed implants. For cortical bone, using an implant decreased von Mises stress with respect to intact values up to 34.4%, with a more pronounced effect at more proximal slices. In the most proximal slice, based on changes in strain energy density, hollow-stemmed implants outperformed solid-stemmed ones through reducing cortical bone volume with resorption potential by 11.7% ± 2.1% (p = .01). For cortical bone in this slice, the percentage of bone with resorption potential for the osteoporotic bone was greater than the healthy bone by 8.0% ± 1.4% using the hollow-stemmed implant (p = .04). These results suggest a small improvement in bone-implant mechanics using hollow-stemmed humeral implants and indicate osteoporosis could exacerbate stress shielding to some extent. The hollow stems maintained adequate strength and using even thinner walls may further reduce stress shielding. After further developing these models, future studies could yield optimized implant designs tuned for varying bone qualities.

Effect of Ulnar Head Offset on Distal Radioulnar Joint Stability

PURPOSE

A complete ulnar head replacement may be indicated in cases of distal radial ulnar joint (DRUJ) dysfunction to address bony pathology in lieu of using a constrained total DRUJ prosthesis. Complete ulnar head implants are simple, but they may be unstable if soft tissue tension is not adequately restored. We hypothesized that incorporating an increased offset in the complete ulnar head replacement would lead to increased tension on the distal oblique interosseous ligament, increased contact force at the DRUJ, and improved joint stability.

METHODS

Using a specially designed jig, we measured instability by comparing displacement under load (stiffness) of the DRUJ in 10 cadaveric specimens under 4 different conditions: (1) intact, (2) native head after excision of the triangular fibrocartilage complex, (3) replacement of the ulnar head with a standard offset ulnar head, and (4) replacement of the ulnar head with an increased offset ulnar head. No soft tissue repair was done. We measured anteroposterior displacement under load with maximum translation of 10 mm or maximum loads of 50 N. We tested all specimens with the forearm positioned in neutral, supination, and pronation.

RESULTS

Excising the triangular fibrocartilage complex decreased the average stiffness of the DRUJ to 46% of the intact state, creating a simulated state of DRUJ instability. Replacing the ulnar head with the standard offset head increased average stiffness to 54% of the intact state. Increasing the ulnar head offset with the simulated total ulnar head replacement increased average stiffness to 77% of the intact state.

CONCLUSIONS

An increased offset ulnar head replacement improves DRUJ stability compared with a standard anatomic offset ulnar head replacement.

CLINICAL RELEVANCE

Understanding DRUJ morphology and offset is important in the treatment of DRUJ arthritis and instability.

Investigating the Effects of Demographics on Shoulder Morphology and Density Using Statistical Shape and Density Modeling

A better understanding of how the shape and density of the shoulder vary among members of a population can help design more effective population-based orthopedic implants. The main objective of this study was to develop statistical shape models (SSMs) and statistical density models (SDMs) of the shoulder to describe the main modes of variability in the shape and density distributions of shoulder bones within a population in terms of principal components (PCs). These PC scores were analyzed, and significant correlations were observed between the shape and density distributions of the shoulder and demographics of the population, such as sex and age. Our results demonstrated that when the overall body sizes of male and female donors were matched, males still had, on average, larger scapulae and thicker humeral cortical bones. Moreover, we concluded that age has a weak but significant inverse effect on the density within the entire shoulder. Weak and moderate, but significant, correlations were also found between many modes of shape and density variations in the shoulder. Our results suggested that donors with bigger humeri have bigger scapulae and higher bone density of humeri corresponds with higher bone density in the scapulae. Finally, asymmetry, to some extent, was noted in the shape and density distributions of the contralateral bones of the shoulder. These results can be used to help guide the designs of population-based prosthesis components and pre-operative surgical planning.

Comparison of proximal humeral bone stresses between stemless, short stem, and standard stem length: a finite element analysis

BACKGROUND

The stem lengths of humeral components used in shoulder arthroplasty vary; however, the literature on these devices is limited. This finite element study investigates the effect of humeral component length on stresses in the proximal humerus.

METHODS

Intact and 3 reconstructed (standard length, short, and stemless implants) finite element models were created from shoulder computed tomography scan data (N = 5). Loading was simulated at varying abduction angles (15°, 45°, and 75°). The average bone stress (represented as a percentage of intact values) was reported at 8 transverse slices. In addition, the overall average change in cortical and trabecular bone stresses was quantified.

RESULTS

Cortical bone stresses in the most proximal slice for the standard (58% ± 12%) and short (78% ± 10%) stem models were significantly reduced compared with the intact (100%) and stemless (101% ± 6%) models (P = .005). These reductions persisted in the second cortical slice for the standard stem compared with the intact, stemless, and short models (P = .025). Interestingly, stresses in the trabecular bone within these proximal slices were significantly elevated when stemless implants were used compared with all other implants (P < .001), regardless of abduction angle.

CONCLUSION

Reducing stem length produced humeral stresses that more closely matched the intact stress distribution in proximal cortical bone. Opposing trends presented in the proximal trabecular bone, probably because of differences in load transfer when shorter stems are used. Accordingly, the results suggest that implant stem length is 1 variable that can be modified in an attempt to better mimic intact bone stresses during humeral component insertion, provided stem fixation is adequate.

Ulnar Head Replacement: 21 Cases; Mean Follow-Up, 7.5 Years

PURPOSE

To report clinical and radiographic outcomes for the Herbert ulnar head prosthesis after a mean of 7.5 years (range, 2.0-12.5 years).

METHODS

We performed 22 Herbert ulnar head prosthesis arthroplasties between 2000 and 2011. Five were primary procedures, and the remaining 17 were done after an average of 2 (range, 1-5) previous operations. The mean age at surgery was 55 years (range, 31-74 years). Follow-up including clinical examination, standardized questionnaires, and radiographic examination was done after mean 7.5 years (range, 2.0-12.5 years) in 21 cases. We used the Disabilities of the Arm, Shoulder, and Hand questionnaire, the Patient-Rated Wrist Evaluation questionnaire, and the Mayo wrist score questionnaire. Pain and satisfaction were evaluated with a 10-cm visual analog scale (VAS). Measurements of range of motion and strength for grip were recorded.

RESULTS

Wrist range of motion was not affected by the arthroplasty except for supination, which significantly improved from 55° to 70°. At follow-up, grip strength averaged 25 kg (range, 10-48 kg) in the operated wrists and 31 kg (range, 8-74 kg) on the contralateral side. Visual analog scale-pain averaged 2.9 (range, 0-8.7) during activity and 1.7 (range, 0-7) at rest. Satisfaction VAS was 8.9 (range, 4.3-10). Five patients had VAS-pain above 5 during activity, and 1 patient was dissatisfied and regretted having undergone arthroplasty. Mean outcomes were 27 (range, 5-50) for Disabilities of the Arm, Shoulder, and Hand measure, 31 (range, 0-90) for the Patient-Rated Wrist Evaluation score, and 71 (range, 30-90) for the Mayo wrist score. One patient was reoperated with capsuloplasty 9 months after the arthroplasty owing to recurrence of painful instability. Full stability was not achieved but the pain resolved. None of the implants showed any radiographic signs of loosening.

CONCLUSIONS

The Herbert ulnar head prosthesis was a safe method of treatment and provided satisfactory midterm results for selected cases of distal radioulnar joint disorders.

CLINICAL RELEVANCE

Increased knowledge of performance for ulnar head implant arthroplasty may aid surgical decision making for distal radioulnar joint disorders.

TYPE OF STUDY/LEVEL OF EVIDENCE

Therapeutic IV.

Determination of remodeling parameters for a strain-adaptive finite element model of the distal ulna

Strain energy–based adaptive material models are used to predict bone resorption resulting from stress shielding induced by prosthetic joint implants. Generally, such models are governed by two key parameters: a homeostatic strain-energy state (K) and a threshold deviation from this state required to initiate bone reformation (s). A refinement procedure has been performed to estimate these parameters in the femur and glenoid; this study investigates the specific influences of these parameters on resulting density distributions in the distal ulna. A finite element model of a human ulna was created using micro-computed tomography (µCT) data, initialized to a homogeneous density distribution, and subjected to approximate in vivo loading. Values for K and s were tested, and the resulting steady-state density distribution compared with values derived from µCT images. The sensitivity of these parameters to initial conditions was examined by altering the initial homogeneous density value. The refined model parameters selected were then applied to six additional human ulnae to determine their performance across individuals. Model accuracy using the refined parameters was found to be comparable with that found in previous studies of the glenoid and femur, and gross bone structures, such as the cortical shell and medullary canal, were reproduced. The model was found to be insensitive to initial conditions; however, a fair degree of variation was observed between the six specimens. This work represents an important contribution to the study of changes in load transfer in the distal ulna following the implementation of commercial orthopedic implants.