Elbow joint biomechanics for preclinical evaluation of total elbow prostheses

Fatigue test evaluation of a customised humeral component for an instrumented total elbow prosthesis and strain validation study

The survival rate of total elbow arthroplasty (TEA) is negatively impacted by the lack of available data on elbow biomechanics. This study developed a modified humeral component for TEA that is purposed to be instrumented to generate real-time 6 degrees of freedom (d.o.f) force and moment data during activities of daily living (ADL). The objectives are twofold: (1) to assess the safety of the modified humeral component under peak anticipated loads in fatigue, and (2) verify the strains measured under physiological loads with strains modelled using finite element analysis (FEA). Four modified titanium alloy humeral components were welded, and fatigue tested at 5 Hz for 5 million cycles under a compressive load of 700 N corresponding to moderate ADL. The strains were measured using triaxial 350 Ω rectangular rosette (45°) strain gauges bonded to three specific locations on the humeral component confirmed through an FE study. The four welded humeral components successfully withstood fatigue conditions and did not deform. The measured and modelled principal strains were confirmed to be highest at the external wall of the lateral cavity, with a percentage difference of <10 %.

Polyethylene wear testing of a nonmechanically linked total elbow replacement

BACKGROUND

The objective of this study was to perform a polyethylene wear test on a nonmechanically linked total elbow arthroplasty implant using a clinically relevant in-vitro elbow wear test methodology that simulated 10 years of use in the light to moderate activity of daily living range.

MATERIALS AND METHODS

The test protocol applied an 80° arc of ulnohumeral motion beginning at 30° shy of full extension and progressing to 110° of flexion. Force was applied at 7° to recreate a valgus load on the elbow. A variable joint load profile at a frequency of 0.5 Hz was employed. The implants were tested for 5 million cycles (mc) in a bovine serum lubricant. Implant component failure was characterized and polyethylene wear was determined gravimetrically.

RESULTS

After 5 mc, the small polyethylene bushing wear rate was 0.56 mg/mc. The medium size wear rate was 0.28 mg/mc. Three large sizes were tested and the average wear rate was 0.39 ± 0.07 mg/mc. No implant failure was identified.

CONCLUSION

The test recreated an in vivo loading environment and measured polyethylene wear rates at specified cycle counts. The test demonstrated less wear than other joint replacements. Further clinical evaluation is necessary to determine if this translates into reduced complications of total elbow replacement associated with wear.

Is headless screw a valid alternative for the fixation of Chevron olecranon osteotomy? A biomechanical comparison of 4 fixation methods

BACKGROUND

Chevron osteotomy is a popular technique for repairing complex distal humerus intra-articular fractures. However, refixation presents challenges like hardware prominence and soft tissue damage. Headless cannulated screws are gaining popularity for fixation due to better bone purchase and less irritation. This study aims to compare different fixation strategies with headless screws for Chevron osteotomy fixation.

METHODS

This study utilized 32 polyurethane foam ulna specimens for biomechanical testing. The Chevron osteotomy was performed with an oscillating saw for all specimens. Four different techniques were used for refixation: modified tension band wire fixation, plate-screw fixation, cannulated screw fixation, and headless cannulated screw fixation. The constructs were mounted to the testing machine, simulating the elbow in 90 degrees of flexion. All specimens were tested under axial traction. Displacements at 350N and 500N and loads at the failure were recorded.

RESULTS

At the load of the implant failure, headless cannulated screw fixation revealed higher force values when compared to the other constructs, and modified tension band wire fixation showed lower force values compared to the other 3 groups (P < .001). The headless cannulated screw fixation group showed no significant differences in osteotomy displacements when tested to a 350N load. There were no significant differences in displacements at a 500N load between the four groups (P = .275).

CONCLUSION

This study suggests that headless cannulated screw fixation is a viable and effective option for Chevron osteotomies. Headless cannulated screw fixation makes it a promising alternative to traditional fixation methods. This technique may be a more reliable fixation technique during daily activities and moderate elbow motions, indicating that it has the potential to succeed.

Difference in daily tasks execution and elbow joint load: a comparison between patients after total elbow arthroplasty and healthy controls

Background

Overloading of the elbow joint is one of the mechanisms responsible for failure of total elbow arthroplasty (TEA). Different activities of daily living (ADL) affect joint loading. However, the alteration in task execution and its impact on joint loads after TEA are not well understood. This study investigates differences between TEA patients and healthy controls in task execution and associated joint loads during simulated ADL.

Methods

TEA patients (n = 7) and healthy controls (n = 18) performed 8 simulated ADL tasks. Using musculoskeletal modeling software (OpenSim), joint angles and moments were calculated and joint power was assessed. A mixed model statistical design was performed to determine group and tasks differences.

Results

TEA patients showed reduced flexion-extension (FE) range of motion (60.6° ± 25.6 vs. 44.9° ± 19.9, P = .003). Interaction effects between groups and tasks for joint load and peak power were observed. Particularly during rising from a chair, patients showed reduced FE moment (5.7 Nm vs. 14.5 Nm, P = .026), varus-valgus moment (6.0 Nm vs. 14.3 Nm, P = .036), and peak power (3.6 Watt vs. 20.1 Watt, P = .036) compared to healthy controls.

Conclusion

TEA patients differ from healthy controls in task execution of ADL tasks regarding the functional elbow FE angle over all 8 ADL tasks and in joint load and peak power for the more straining tasks. The power plots visualizes differences in movement strategy that are of interest for future research on possible training of TEA patients, or prosthesis design, aimed to improve ADL function and enhance prosthesis survival rates.

The Effect of Arm Abduction and Forearm Muscle Activation on Kinematics During Elbow Flexion

Purpose

As the elbow flexes with the arm at the side (0° humerothoracic abduction, HTA), it loses its valgus carrying angle. When the arm is abducted to 90° HTA, a varus torque tensions the lateral ligaments. Our purpose was to quantify the effect of abduction on elbow kinematics during active motion and the effect of lateral forearm muscle activation. We hypothesized that arm abduction would increase elbow varus angulation throughout flexion, and lateral forearm muscle activation would decrease varus angulation.

Methods

A dynamic elbow testing apparatus was employed in six human cadaver arms at two levels of arm abduction, 0° and 90° HTA. Six electromechanical actuators simulated muscle action, whereas joint position was measured to quantify the relationship between the forearm and humerus as the elbow was actively flexed.

Results

All elbows maintained greater varus angle with the arm at 90° HTA compared with 0° HTA, significant at 60° flexion, 4.3° versus 3.4°, 90° flexion, 8.0° versus 6.8°, and 120° flexion, 10.5° versus 8.9°. The abducted elbow demonstrated less varus angle when the lateral stabilizers were activated. A significant difference was found at 30° flexion, 0.9 versus 1.5, 60° flexion, 3.8 versus 4.3, and 90° flexion, 7.6 versus 8.0.

Conclusions

Elbow joint coronal plane kinematics were influenced by abduction of the arm to 90° HTA, and greater elbow varus angles were found throughout flexion when compared with the arm at side position (0° HTA). In addition, activation of lateral forearm muscles (90° HTA + Lat Stab) decreased elbow varus angulation throughout flexion.

Clinical relevance

Understanding the effect of varus torque on elbow biomechanics and the degree to which these effects are countered through dynamic stabilization may assist in arthroplasty and ligamentous reconstruction designs.

Postoperative Weightlifting Restrictions Following Elbow Arthroplasty: A Survey of Italian Society of Shoulder and Elbow Surgery Members

Background: Total elbow arthroplasty (TEA) has evolved over time from a salvage procedure to a successful treatment of end-stage primary and secondary arthritis. However, the aseptic loosening and the associated reduced survival rate are still concerning. Thus, TEA is typically contraindicated in young and active patients where high-demand activities would promote aseptic loosening. For this reason, postoperative weightlifting limitations are often suggested, yet there is no consensus. The aim of this survey was to collect and analyze the current practice concerning the weightlifting restrictions following elbow arthroplasties among members of the Italian Society of Shoulder and Elbow Surgery (Società Italiana Chirurgia Spalla e Gomito, SICSeG). Methods: An online survey on the lifting restrictions after elbow arthroplasties was submitted to all members of the SICSeG. Results: In total, 36 members of the Italian society completed the survey. Only five consultants (13.8%) have experience with all the implants analyzed, of whom only three have experience with more than 10 implants per year. Concerning the comprehensive number of elbow arthroplasties performed per year, most of the respondents (45.7%) reported fewer than five surgeries per year, whereas only two surgeons claimed more than 20 procedures. Of the 36 respondents, 32 (88.9%) reported lifelong lifting limitations after linked TEA. In detail, these lifting restrictions were 10 lb in 14 responders (38.9%) and 5 lb in 15 responders (41.7%) performing linked TEA. A lifelong lifting limitation gradually decreased after unlinked TEA and hemiarthroplasty (HA) being advised by 82.8% (24/36) and 64.5% (20/36), respectively. Conclusions: To increase implant longevity, most Italian surgeons advise lifting restrictions after TEA. More than 80% of the responders agreed with suggesting lifelong limitations, but a greater variability was found in the amount of weight to which the patients are restricted. Currently, the lack of consensus on the optimal weightlifting restrictions after elbow replacements emphasizes the need for more studies focusing on elbow joint loading during different activities of daily life to improve implant survival rates.

Lifting limitations following elbow arthroplasty: A survey of British Elbow and Shoulder Society members

The primary aim of this survey was to capture current practice with regard to lifting limitations following elbow arthroplasty. An online survey was emailed to all members of the British Elbow and Shoulder Society. All consultant respondents who reported performing elbow arthroplasty were asked about their advice for post-operative lifting limitations. In total, 115 surveys were completed: 55 consultants reported performing linked total elbow arthroplasty, 18 reported performing unlinked total elbow arthroplasty and 44 reported performing distal humeral hemiarthroplasty. The majority of elbow consultants advise a lifelong lifting limitation following linked and unlinked total elbow arthroplasty (78% and 61% respectively). There was variation in the weight specified for lifelong lifting limitations, the median weight restriction in linked total elbow arthroplasty was 5 lb, and in unlinked total elbow arthroplasty was 10 lb (range 1-20 lb). In total, 13% of consultants performing linked total elbow arthroplasty and 33% of consultants performing unlinked total elbow arthroplasty do not advise any lifelong lifting limitations post-operatively. In a perceived attempt to prolong implant longevity, most surgeons recommend lifelong lifting limitations following total elbow arthroplasty. There is variation in the weight restriction advised by consultant elbow surgeons. Currently the optimal weight restriction to maximise implant longevity is not known and further work needs to be done to understand the true relationship between activity, loading and implant failure.

An Ergonomic Golf Grip Leads to Lower Forearm Muscle Activity - A Prospective Case Series of 30 Right-Handed Amateur and Professional Golfers

BACKGROUND

The elbow is a common site for overuse injuries in golfers. Tendinopathies, such as medial and lateral epicondylitis, are frequently diagnosed in amateur and professional golfers. The aim of our study was to determine the effect of an ergonomic golf grip on forearm muscle activity during the five phases of the golf swing.

METHODS

Thirty right-handed golfers with a mean age of 32 years (range, 18-70 years) and a mean handicap of 15 (range, 0-43) performed 10 golf swings with a standard and ergonomic golf grip respectively. The mean and maximum muscle activity of the Musculus (M.) extensor carpi radialis brevis (ECRB), M. flexor carpi ulnaris (FCU), M. pronator teres (PT) and M. biceps brachii (BB) of the lead and trail arms were assessed during the five phases of the golf swing using surface electromyography (EMG). Subgroup analyses were performed regarding sex, playing ability (handicap < 10 vs. ≥10), weekly playing time (≤ 5 h, 5-20 h, > 20 h) and preexisting elbow pain during golfing (VAS < 2 vs. VAS ≥ 2). Significance was set at p < 0.05.

RESULTS

An ergonomic golf grip resulted in a reduction in muscle activity in at least one but up to three consecutive phases of the golf swing for the ECRB, FCU and PT of the lead arm and for the PT of the trail arm. Amateurs, a playing time < 20 h per week and golfers without preexisting elbow pain were factors that were associated with greater reductions in muscle activity.

CONCLUSION

Forearm muscle activity can be decreased using an ergonomic golf grip, indicating the possible role of an ergonomic golf grip as a preventive measure against overuse syndromes such as medial and lateral epicondylitis.

TRIAL REGISTRATION NUMBER

This study was retrospectively registered at the German Clinical Trials Register DRKS-ID: DRKS00033732 (01/03/2024).

How Does the Subchondral Bone Density Distribution of the Distal Humerus Change Between Early and Advanced Stages of Osteoarthritis?

BACKGROUND

The distribution of subchondral bone density in a joint represents stress that is applied to the joint. Knowing this information is important for understanding the pathophysiology of osteoarthritis (OA). In the elbow, however, this has not been studied before.

QUESTIONS/PURPOSES

(1) Is advanced-stage elbow OA associated with more radially distributed subchondral bone density than earlier stages? (2) What demographic (age and sex) and radiographic (osteophyte location and carrying angle) factors are associated with increased radial shift in subchondral bone density?

METHODS

Between March 2001 and December 2021, we treated 301 patients for elbow OA. We considered patients with plain radiographs and conventional CT scans as potentially eligible. Thus, 68% (206 patients) were eligible; a further 27% (80 patients) were excluded because of a history of any injury or surgery or known inflammatory joint disease, leaving 42% (126 patients) for analysis here. Their mean ± standard deviation age was 60 ± 10 years. Early OA with minimal joint space narrowing and osteophyte formation was found in 33% (42 of 126) of patients, and advanced OA was found in the remaining 67% (84 of 126). Three-dimensional distal humerus subchondral bone models were derived from CT images, and in the central intra-articulating portion, we measured the subchondral bone density in two different sites: where it articulates with the radius (SBD rad ) and with the ulna (SBD ulna ). We further defined the SBD ratio as the percent ratio of SBD rad to SBD ulna . We also evaluated osteophyte severity based on its size at the radiocapitellar and ulnotrochlear joints, and alignment through measuring the carrying angle on radiographs. To assess interobserver reliability, two orthopaedic surgeons took measurements independently from each other. All measurements had excellent intraoberver and interobserver reliabilities. Then, we compared the subchondral bone parameters between early and advanced OA and performed a multivariable analysis of the factors associated with subchondral bone parameters, including age, sex, osteophyte location, and carrying angle.

RESULTS

Radial versus ulna subchondral bone density (SBD ratio ) was modestly higher in patients with advanced OA (118% ± 17%) than in patients with early OA (109% ± 17%, mean difference 9% [95% CI 2.3% to 15.3%]; p = 0.01). With increasing radial deviation in subchondral bone density, cubitus valgus had a modest association (β = 0.46 ± 0.23; p = 0.04) and severe osteophytes at the radiocapitellar joint had a large association (β = 9.51 ± 3.06; p = 0.002).

CONCLUSION

According to subchondral bone density distribution, stress concentration was more radially deviated in patients with the advanced stages of elbow OA than in those with the early stages. We also found that an increase in carrying angle is associated with radial deviation of stress. A future study that examines longitudinal changes in the subchondral bone density might be required to confirm changes in stress concentration with OA progression.

CLINICAL RELEVANCE

This study gives us insight into the potential pathophysiology of elbow OA in relation to elbow alignment. Although debridement of osteophytes in the ulnotrochlear joint is the most frequently performed procedure in patients with advanced elbow OA, our finding suggests that some patients with an increased carrying angle might benefit from management of the radiocapitellar joint as well, or from being informed of the future development of OA in the radiocapitellar joint, because stress at this site can be increased with the advancement of OA.

Weak Points of Double-Plate Stabilization Used in the Treatment of Distal Humerus Fracture through Finite Element Analysis

BACKGROUND

Multi-comminuted, intra-articular fractures of the distal humerus still pose a challenge to modern orthopedics due to unsatisfactory treatment results and a high percentage (over 50%) of postoperative complications. When surgical treatment is chosen, such fractures are fixed using two plates with locking screws, which can be used in three spatial configurations: either parallel or one of two perpendicular variants (posterolateral and posteromedial). The evaluation of the fracture healing conditions for these plate configurations is unambiguous. The contradictions between the conclusions of biomechanical studies and clinical observations were the motivation to undertake a more in-depth biomechanical analysis aiming to indicate the weak points of two-plate fracture stabilization.

METHODS

Research was conducted using the finite element method based on an experimentally validated model. Three variants of distal humerus fracture (Y, λ, and H) were fixed using three different plate configurations (parallel, posterolateral, and posteromedial), and they were analyzed under six loading conditions, covering the whole range of flexion in the elbow joint (0-145°). A joint reaction force equal to 150 N was assumed, which corresponds with holding a weight of 1 kg in the hand. The biomechanical conditions of bone union were assessed based on the interfragmentary movement (IFM) and using criteria formulated by Steiner et al. Results: The IFMs were established for particular regions of all of the analyzed types of fracture, with distinction to the normal and tangential components. In general, the tangential component of IFM was greater than normal. A strong influence of the elbow joint's angular position on the IFM was observed, with excessive values occurring for flexion angles greater than 90°. In most cases, the smallest IFM values were obtained for the parallel plaiting, while the greatest values were obtained for the posteromedial plating. Based on IFM values, fracture healing conditions in particular cases (fracture type, plate configuration, loading condition, and fracture gap localization) were classified into one of four groups: optimal bone union (OPT), probable union (PU), probable non-union (PNU), and non-union (NU).

CONCLUSIONS

No plating configuration is able to ensure distal humerus fracture union when the full elbow flexion is allowed while holding a weight of 1 kg in the hand. However, flexion in the range of 0-90° with such loadings is acceptable when using parallel plating, which is a positive finding in the context of the early rehabilitation process. In general, parallel plating ensures better conditions for fracture healing than perpendicular plate configurations, especially the posteromedial version.

Elbow joint loads during simulated activities of daily living: implications for formulating recommendations after total elbow arthroplasty

BACKGROUND

Overloading of the elbow joint prosthesis following total elbow arthroplasty can lead to implant failure. Joint moments during daily activities are not well contextualized for a prosthesis's failure limits, and the effect of the current postoperative instruction on elbow joint loading is unclear. This study investigates the difference in elbow joint moments between simulated daily tasks and between flexion-extension, pronation-supination, and varus-valgus movement directions. Additionally, the effect of the current postoperative instruction on elbow joint load is examined.

METHODS

Nine healthy participants (age 45.8 ± 17 years, 3 males) performed 8 tasks; driving a car, opening a door, rising from a chair, lifting, sliding, combing hair, drinking, emptying cup, without and with the instruction "not lifting more than 1 kg." Upper limb kinematics and hand contact forces were measured. Elbow joint angles and net moments were analyzed using inverse dynamic analysis, where the net moments are estimated from movement data and external forces.

RESULTS

Peak elbow joint moments differed significantly between tasks (P < .01) and movement directions (P < .01). The most and least demanding tasks were, rising from a chair (13.4 Nm extension, 5.0 Nm supination, and 15.2 Nm valgus) and sliding (4.3 Nm flexion, 1.7 Nm supination, and 2.6 Nm varus). Net moments were significantly reduced after instruction only in the chair task (P < .01).

CONCLUSION

This study analyzed elbow joint moments in different directions during daily tasks. The outcomes question whether postoperative instruction can lead to decreasing elbow loads. Future research might focus on reducing elbow loads in the flexion-extension and varus-valgus directions.

Active Motion Laboratory Test Apparatus for Evaluation of Total Elbow Prostheses

Purpose

The goal of this study was to develop a dynamic elbow testing apparatus that reproduces active joint motion at different shoulder positions to quantify the capabilities of total elbow arthroplasty designs.

Methods

We designed a testing apparatus to create active cyclic elbow joint motion in human cadaveric and sawbones composite upper extremities. Two pneumatic actuators recreated humerus-originating muscles while rubber bands simulated forearm muscle action. Arthroplasty durability was quantified through laxity assessment at predetermined cyclic loading intervals.

Results

Humeral forces were recorded in three specimens to generate active elbow motion at different degrees of shoulder abduction. The laxity in varus and valgus was measured as deflection between two fixed markers.

Conclusions

In vitro simulation of elbow biomechanics through active cyclic elbow motion at different degrees of shoulder abduction may characterize in vivo performance of total elbow arthroplasty.

Clinical relevance

Quantifying total elbow arthroplasty stability after cyclic loading in different shoulder positions may assist preclinical evaluation of arthroplasty designs.

A Dynamic Elbow Testing Apparatus for Simulating Elbow Joint Motion in Varying Shoulder Positions

Purpose

To develop and evaluate the capabilities of a dynamic elbow testing apparatus that simulates unconstrained elbow motion throughout the range of humerothoracic (HTA) abduction.

Methods

Elbow flexion was generated by six computer-controlled electromechanical actuators that simulated muscle action, while six degree-of-freedom joint motion was measured using an optical tracking device. Repeatability of joint kinematics was assessed at four HTA angles (0°, 45°, 90°, 135°) and with two muscle force combinations (A1-biceps brachialis, brachioradialis and A2-biceps, brachioradialis). Repeatability was determined by comparing kinematics at every 10° of flexion over five flexion-extension cycles (0° to 100°).

Results

Multiple muscle force combinations can be used at each HTA angle to generate elbow flexion. Trials showed that the testing apparatus produced highly repeatable joint motion at each HTA angle and with varying muscle force combinations. The intraclass correlation coefficient was greater than 0.95 for all conditions.

Conclusions

Repeatable smooth cadaveric elbow motion was created that mimicked the in vivo situation.

Clinical relevance

These results suggest that the dynamic elbow testing apparatus can be used to characterize elbow biomechanics in cadaver upper extremities.

Testing of Novel Total Elbow Prostheses Using Active Motion Experimental Setup

PURPOSE

The goal of this study was to test a novel uncemented and unconstrained total elbow arthroplasty (Kaufmann total elbow) design that is stabilized through a ligament reconstruction.

METHODS

We quantified the implant stability after 25,000 cycles, which represents the time between implantation and when ligament and bone healing has occurred. We used an active motion experimental setup that applies tendon loads via pneumatic cylinders and reproduces the forearm-originating dynamic stabilizers of the elbow. The novel total elbow arthroplasty was actuated for 5,000 full flexion-extension cycles at 5 different shoulder positions. Four Sawbones and 4 cadaver elbows were employed. Angular laxity and implant stability were recorded prior to testing and after each 5,000-loading cycle.

RESULTS

Four Sawbones and 4 cadaver elbows were implanted with the uncemented total elbow arthroplasty and did not demonstrate fixation failure or substantial laxity after 25,000 cycles of loading imparted at different shoulder positions.

CONCLUSIONS

Our findings demonstrate that the Kaufmann total elbow replacement implanted into cadaver and Sawbones specimens did not exhibit fixation failure or excessive laxity after 25,000 cycles.

CLINICAL RELEVANCE

An uncemented, nonmechanically linked total elbow arthroplasty that gains component fixation using intramedullary screws and employs a ligament reconstruction to stabilize the elbow has the potential to be a valuable management option, particularly in younger patients.

Chronic elbow instability in adults: The why, when and how of ligament reconstruction

Chronic elbow instability in adults mainly consists of valgus instability and posterolateral instability. They most often occur because of ligament damage following elbow dislocation but can also occur due to repeated microtrauma. The aim of ligament stabilization surgery is always the same: recreate the anatomy and function of the original ligament. Extensive knowledge of the elbow's anatomical structures and biomechanics is crucial to understanding why the elbow is unstable and how to treat it. In this review, we will explain how elbow instability develops, what types of grafts are available and which reconstruction techniques can be used for posterolateral or valgus instability. LEVEL OF EVIDENCE: 3.

Tension-band wiring of displaced stable olecranon fractures with Eyelet-pins in the elderly: A series of 17 cases

BACKGROUND

Tension-band wiring (TBW) and plate fixation are commonly used to fix displaced olecranon fractures. However, the high incidence of complications in the elderly, such as wound breakdown, infection, and loss of reduction, remain a concern for orthopaedic surgeons. Furthermore, patients frequently suffer from removal of the hardware. Even so, the operation seems to be indicated for independent elderly patients to return to their former activities of daily living.

HYPOTHESIS

TBW of displaced stable olecranon fractures with Eyelet-pins in the independent elderly reduce the incidence of complications and allow early elbow joint exercise to keep their former activities.

PATIENTS AND METHODS

We operated on the displaced stable olecranon fractures of 17 independent patients aged ≥ 70 using TBW with two Eyelet-pins. Eyelet-pins have an eyelet at the trailing end to prevent pin migration by passing a soft wire through it. The patients were reviewed clinically and radiologically at 2, 6, 12, and 24 weeks, and clinically at 1 year after the surgery.

RESULTS

All fractures were united within 12 weeks, and the anatomic reduction was maintained. Mean radiographic proximal migration of the Eyelet-pins was 0.4mm (0.1 ∼ 1.2mm). Mean active elbow flexion was 136° (115° ∼ 145°) and extension 6.2° (0° ∼ 30°). Two patients had mild local pain and pain on motion at the tip of the eyelet. No patient required removal of the hardware. Other complications, such as superficial or deep wound infections, and neurological symptoms or signs, were not seen. All patients were able to maintain their former activities of daily living.

CONCLUSION

TBW with Eyelet-pins for displaced stable olecranon fractures is useful for independent elderly patients to reduce the incidence of complications and to maintain their former activities of daily living without removal of the hardware.

LEVEL OF EVIDENCE

IV; single-centre retrospective study.

3D printing-assisted fabrication of patient-specific antibacterial radial head prosthesis with high periprosthetic bone preservation

We present a novel fabrication and surgical approach for anatomical reconstruction of a fractured radial head using patient-specific radial head prosthesis made of polymethylmethacrylate (PMMA) bone cement. To this end, the use of PMMA bone cement for prosthesis fabrication was initially investigated using computational modeling and experimental methods. The radial head prosthesis was fabricated through casting of PMMA bone cement in silicone mold in the operation room before implantation. To enhance the precision of bony preparation for replacement of the radial head, patient-specific surgical guide for accurate resection of the radial neck with the desired length was developed. Post-surgical clinical examinations revealed biomechanical restoration of elbow function, owing to the use of patient-specific radial head prosthesis and surgical guide. Importantly, follow-up radiographs after a mean follow-up of 18 months revealed bone preservation at the bone-prosthesis interface without any signs of erosion of the capitellum. Taken together, our method demonstrated the safety and efficacy of the PMMA radial head prosthesis in restoring elbow biomechanics. This also provides a very safe and cost-effective method for making various patient-specific prostheses with localized antibacterial delivery and close mechanical properties to native bone for improved periprosthetic bone regeneration.

Quantitative ultrahigh-molecular-weight polyethylene wear in total elbow retrievals

BACKGROUND

The purpose of this study was to evaluate ultrahigh-molecular-weight polyethylene (UHMWPE) wear and damage from retrieved total elbow arthroplasty components and compare in vivo wear with wear produced in vitro.

METHODS

Explanted total elbow components were collected at revision surgery. UHMWPE damage was characterized visually, whereas penetration and wear were quantified using micro-computed tomography and gas pycnometry. Volumetric wear rates were compared with historical hip data, and wear data were compared with reported in vitro wear test data.

RESULTS

Humeral bushing damage primarily occurred in the form of burnishing, scratching, and pitting at the articular face in the region of contact with the ulnar component. Wear of the ulnar bushings was concentrated on the edge of the component at the point of contact with the axis pin. Pitting and embedded debris were dominant damage modes, in addition to burnishing and delamination. Backside wear was negligible. The median linear penetration rates of the lateral, medial, and ulnar bushings were 0.14 mm/yr (range, 0.01-0.78 mm/yr), 0.12 mm/yr (range, 0.03-0.55 mm/yr), and 0.11 mm/yr (range, 0.01-0.69 mm/yr), respectively. The volumetric wear rates of the lateral, medial, and ulnar bushings were 5.5 mm³/yr (range, 0.7-37.2 mm³/yr), 5.9 mm³/yr (range, 0.6-25.5 mm³/yr), and 5.5 mm³/yr (range, 1.2-51.2 mm³/yr), respectively.

CONCLUSIONS

The observed wear rates were similar to those reported in well-functioning total hip replacement patients with conventional UHMWPE bearings. We found limitations in reported in vitro testing resulting in wear that was not consistent with our retrieval data. We recommend further investigation to clinically validate in vitro simulation to provide appropriate loading protocols for elbow wear simulation.

Elbow Biomechanics: Bony and Dynamic Stabilizers

The elbow positions the hand in a stable manner relative to the trunk while allowing flexion and extension as well as forearm rotation at varying shoulder positions. Its ability to perform this task without joint subluxation is accomplished through a combination of bony congruency, ligamentous restraint, and dynamic stabilization. This article reviews the bony and dynamic contributors to elbow stability and kinematics. Bony stability is conferred through the morphology of the humeroulnar, humeroradial, and radioulnar joints. Depending on the arm position relative to the trunk and the degree of elbow flexion, the bony contribution will vary. Dynamic elbow stabilizers confer stability through the activation of various muscles that cross the elbow. These forces help resist valgus and varus forces and may also increase bony stability by generating compressive forces. The goal of this article is to review the literature surrounding the biomechanics of bony and dynamic stabilizers of the elbow while drawing clinically relevant biomechanical observations.

Total Elbow Arthroplasty: Elbow Biomechanics and Failure

Total elbow arthroplasty (TEA) has lower revision-free survivorship than other major joint replacement. Despite this, elbow replacement has remained popular. Surgical technique for TEA destabilizes the elbow by removing the medial and lateral collateral ligaments and, frequently, the radiocapitellar articulation. Current semiconstrained implants aim to allow for physiological varus and valgus motion by employing a sloppy hinge; however, over time, these designs fail owing to nonanatomic force transmission. Nonanatomic force transmission results from ligament release, force transmission primarily to the humeral and ulnar shafts, and radial head resection. These altered biomechanics may be the source of complications seen in semiconstrained arthroplasty. These complications perpetuate the cycle of failure and ultimately have poor salvage options.

Biomechanical Evaluation of Standard Versus Extended Proximal Fixation Olecranon Plates for Fixation of Olecranon Fractures

Background: Small olecranon fractures present a significant challenge for fixation, which has resulted in development of plates with proximal extension. Olecranon-specific plates with proximal extensions are widely thought to offer superior fixation of small proximal fragments but have distinct disadvantages: larger dissection, increased hardware prominence, and the increased possibility of impingement. Previous biomechanical studies of olecranon fracture fixation have compared methods of fracture fixation, but to date there have been no studies defining olecranon plate fixation strength for standard versus extended olecranon plates. The purpose of this study is to evaluate the biomechanical utility of the extended plate for treatment of olecranon fractures. Methods: Sixteen matched pairs of fresh-frozen human cadaveric elbows were used. Of the 16, 8 matched pairs received a transverse osteotomy including 25% and 8 including 50% of the articular surface on the proximal fragment. One elbow from each pair was randomly assigned to a standard-length plate, and the other elbow in the pair received the extended-length plate, for fixation of the fracture. The ulnae were cyclically loaded and subsequently loaded to failure, with ultimate load, number of cycles, and gap formation recorded. Results: There was no statistically significant difference between the standard and extended fixation plates in simple transverse fractures at either 25% or 50% from the proximal most portion of the articular surface of the olecranon. Conclusion: Standard fixation plates are sufficient for the fixation of small transverse fractures, but caution should be utilized particularly with comminution and nontransverse fracture patterns.

Development and Validation of a Method for Preclinical Durability Evaluation of Linked Semiconstrained Total Elbow Replacement Prostheses

Total elbow replacement (TER) is a clinically successful procedure yet isolated, gross mechanical complications associated with implant durability persist. The objectives of this study were to (1) develop a clinically relevant in vitro methodology to replicate the reported damage modes and (2) demonstrate durability improvements of a next-generation linked, semiconstrained design. Two TER prostheses were tested on a biaxial test frame at 1.4 Hz in 37 ± 3° deionized water through 0° to 130° flexion/extension at various load levels simulating high demand, posttraumatic patients until either component failure or run out to 200 000 cycles. The damage patterns of tested components were qualitatively compared to retrieved components to establish the clinical validity of the methodology. The run out load of design 1 was equivalent to 100 N weight in hand (WIH). Specimens tested at higher load levels exhibited multimodal damage consistent in appearance with the clinical literature. The minimum run out load of design 2 was 110 N WIH with no significant damage observed on the components. The methodology developed here was shown to reproduce the clinical damage modes associated with TER in high demand, posttraumatic patients. The method was able to distinguish performance differences within and between 2 different linked, semiconstrained designs.

Effects of axial forearm instability on force transmission across the elbow

BACKGROUND

The interosseous membrane (IOM) and distal radioulnar joint (DRUJ) provide axial stability to the forearm. Our hypothesis was that injury to these structures alters force transmission through the elbow.

METHODS

A custom-designed apparatus that applies axial loads from the wrist to the elbow was used to test 10 cadaveric upper limbs under the following simulated conditions (1) intact, (2) DRUJ injury, (3) IOM injury, or (4) IOM + DRUJ injury. IOM injury was simulated by osteotomies of the IOM attachment to the radius, and DRUJ injury was simulated by distal ulnar oblique osteotomy. We applied 160 N of axial force during cyclic and functional range of forearm rotation (40^o pronation/40^o supination), and force, contact pressure, and contact area through the elbow joint were measured simultaneously.

RESULTS

The force across the radiocapitellar joint was significantly higher in the IOM + DRUJ injury and the IOM injury groups than in the intact and DRUJ injury groups. The mean force across the radiocapitellar joint was not significantly different between the intact and DRUJ injury groups or between the IOM + DRUJ injury and the IOM injury groups. Forces across the ulnohumeral joint showed an inverse pattern to those in the radiocapitellar joint.

CONCLUSIONS

These findings suggest that injury to the IOM contributes more to the disruption of the normal distribution of axial loads across the elbow than injury to the DRUJ.

Physiological Loading of the Coonrad/Morrey, Nexel, and Discovery Elbow Systems: Evaluation by Finite Element Analysis

PURPOSE

Wear of polyethylene bearings represents a limiting factor in the long-term success of total elbow prostheses. Bearing stress is 1 factor contributing to accelerated wear. Physiological loading of total elbow prostheses and implant design influence upon bearing stresses have not been well described. This study evaluates bearing stresses in 3 commercially available implant designs under loads associated with daily living.

METHODS

Motion tracking from a healthy volunteer helped establish a musculoskeletal model to simulate flexor and extensor muscle activation at 0°, 45°, and 90° of shoulder abduction with a 2.3-kg weight in hand-forces and moments were measured at the elbow. Resulting physiological joint reaction forces and moments were applied to finite element models of 3 total elbow bearing designs (Coonrad/Morrey, Nexel, and Discovery) to evaluate contact area and polyethylene stresses.

RESULTS

Increasing shoulder abduction resulted in minimal changes to the elbow joint reaction force but greater joint moments. All implants showed greater peak stresses with increasing shoulder abduction-elbow varus. Discovery and Nexel achieved greater contact area (23% vs > 100%) and demonstrated up to 39% lower peak polyethylene stresses compared with the Coonrad/Morrey design.

CONCLUSIONS

Shoulder abduction results in a varus moment at the elbow. Newer bearing designs (Nexel and Discovery) provide a combination of higher contact area, improved load sharing, reduced edge loading, and lower stresses through elbow range of motion when compared with a cylindrical hinge-bearing design (Coonrad/Morrey).

CLINICAL RELEVANCE

Although the Coonrad/Morrey is a clinically successful prosthesis, our physiological loading model shows that Discovery and Nexel provide greater contact area, better load sharing and lower peak stresses. This may lead to a decrease in polyethylene wear rates and the eventual risks of osteolysis and aseptic loosening. Further studies are needed to determine how these findings translate clinically.

Joint Contact Changes With Undersized Prosthetic Radial Heads

OBJECTIVE

To evaluate the effect of intentional undersizing of prosthetic radial head implant diameters on joint contact pressures.

METHODS

Eight fresh-frozen cadaveric elbows were aligned in neutral extension and loaded with 100 N using a custom testing apparatus. Radiocapitellar contact pressures were recorded using a Tekscan thin-film pressure sensor. Prosthetic radial head replacement was performed with 2 prostheses: the Anatomic Radial Head and the Evolve Proline Radial Head prostheses. Each design was sized according to the manufacturer's recommendations and then again using 2-mm smaller radial heads.

RESULTS

Average and peak pressures were significantly higher with the Evolve than the Anatomic prostheses (P < 0.03 and 0.02, respectively). Peak pressures decreased from 4.2 ± 0.5 MPa to 2.9 ± 0.3 MPa for the Anatomic Radial Heads and from 5.6 ± 0.5 MPa to 3.9 ± 0.6 MPa when the Evolve Radial Heads were undersized by 2 mm. The mean pressures of the Anatomic Radial Heads (1.4 ± 0.1 MPa) did not change significantly with undersizing (1.3 ± 0.1 MPa, P = 0.12), whereas the mean pressures of the Evolve Radial Heads (1.6 ± 0.1 MPa) were significantly reduced with undersizing (1.4 ± 0.1 MPa, P < 0.02).

CONCLUSION

Both mean and peak pressures were initially high for the Evolve Radial Head sized based on the short axis diameter and were improved with further undersizing by 2 mm. Peak, but not mean, contact pressures were improved by undersizing the Anatomic prosthesis based on the long axis diameter.

CLINICAL RELEVANCE

These findings support the clinical recommendation of some surgeons to undersize the Evolve prosthesis by 2-mm smaller diameter than the current manufacturer's suggestion and give reason to consider doing the same for the Anatomic prosthesis.

[Elbow prostheses in rheumatic diseases]

When the elbow is destroyed due to rheumatic diseases, the joint can be replaced by a prosthesis and total arthroplasty. Improved pharmaceutical treatment for rheumatic diseases has, however, reduced the number of implantations in these patients. Reported 10-year survival rates of the implant currently achieve 81-90%.; however, due to limited long-term survival of the implant and high complication rates, total elbow arthroplasty should still be used with caution. Continuous technical improvements in the available prostheses and in surgical techniques could lead in the future to a decline in complications, such as aseptic loosening and infections.

Comparing damage on retrieved total elbow replacement bushings with lab worn specimens subjected to varied loading conditions

Complication rates following total elbow replacement (TER) with conventional implants are relatively high due to mechanical failure involving the UHMWPE bushings. Unfortunately, there are no standardized pre-clinical durability testing protocols for assessing the durability of TER components. This study examines the damage observed on retrieved humeral bushings, and then uses in vitro durability testing with two different loading protocols to compare resulting damage. Damage on 25 pairs of retrieved humeral bushings was characterized using micro-computed tomographic imaging techniques. The damage was compared with that of in vitro test specimens which were subjected to 200 K cycles of either high joint reaction force (high JRF) or high varus moment (high VM) loading. Material removal (mass loss) from bushing components was measured using gravimetric techniques. Thinning was less for retrieved bushings which were still assembled in their humeral component, versus bushings which were loose (0.3 ± 0.3 mm vs. 0.6 ± 0.3 mm, p = 0.02). Comparing in vitro test specimens, thinning due to high VM loading was 0.9 ± 0.3 mm, versus 0.2 ± 0.0 mm for high JRF loading (p = 0.08); however, the actual material removal rates from the humeral bushings were not different between the two protocols (48 ± 5 mm³ /Mc vs. 43 ± 2 mm³ /Mc, p = 1). Neither loading protocol could produce damage patterns fully representative of the spectrum of damage patterns observed on clinical retrievals. Pre-clinical testing should employ multiple loading protocols to characterize implant performance under a broader spectrum of usage. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1998-2006, 2018.

Development of a hybrid computational/experimental framework for evaluation of damage mechanisms of a linked semiconstrained total elbow system

BACKGROUND

Long-term durability of total elbow arthroplasty (TEA) is a concern, and bearing wear or excessive deformations may necessitate early revision. The current study used experimental wear testing and computational finite element modeling to develop a hybrid computational and experimental framework for the evaluation of TEA damage mechanisms.

METHODS

Three Coonrad-Morrey (Zimmer-Biomet Inc., Warsaw, IN, USA) TEA implants were used for experimental wear testing for 200,000  cycles. Gravimetric measurements were performed before and after the tests to assess the weight change caused by wear. A finite element model of the implant was also developed to analyze ultrahigh-molecular-weight polyethylene (UHMWPE) damage.

RESULTS

High localized contact pressures caused visible creep and plastic flow, deforming bushings and creating unintended UHMWPE-on-UHMWPE contact surfaces where considerably high wear rates were observed. Average experimentally measured vs. model-predicted wear was 9.5 ± 1.0 vs. 14.1 mg for the of the medial bushing, 8.5 ± 1.0 vs. 13.9 mg for the lateral humeral bushing, and 34.1 ± 0.7 vs. 36.9 mg for the ulnar bushings, respectively. Model predicted contact stresses on the surfaces of bushings were substantially higher than the yield limit of conventional UHMWPE (87 MPa for the humeral bushings and 83 MPa for the ulnar bushing).

CONCLUSIONS

Our study discovered that unintended wear at UHMWPE-UHMWPE contact surfaces, "fed" by excessive plastic flow may, in fact, be of more concern than wear that occurs at the intended metal-UHMWPE contact interfaces. Furthermore, formation of high localized contact stresses much above the yield limit of UHMWPE is another likely contributor to bushing failure for this implant.

An In Vitro Study of the Role of Implant Positioning on Ulnohumeral Articular Contact in Distal Humeral Hemiarthroplasty

PURPOSE

To investigate the effect of implant positioning on ulnohumeral contact using patient-specific distal humeral (DH) implants.

METHODS

Seven reverse-engineered DH implants were manufactured based on computed tomography scans of their osseous geometry. Native ulnae were paired with corresponding native humeri and custom DH implants in a loading apparatus. The ulna was set at 90° of flexion and the humeral component (either native bone or reverse-engineered implant) was positioned from 5° varus to 5° valgus in 2.5° increments under a 100-N compressive load. Contact with the ulna was measured with both the native distal humerus and the reverse-engineered DH implant at all varus-valgus (VV) angles, using a joint casting method. Contact patches were digitized and analyzed in 4 ulnar quadrants. Output variables were contact area and contact pattern.

RESULTS

Mean contact area of the native articulation was significantly greater than with the distal humeral hemiarthroplasty (DHH) implants across all VV positions. Within the native condition, contact area did not significantly change owing to VV angulation. Within the DHH condition, contact area also did not significantly change owing to VV angulation. Conversely, in the DHH condition, contact pattern did significantly change. Medial ulnar contact pattern was significantly affected by VV angulation. Lateral ulnar contact was variably affected, but generally decreased as well.

CONCLUSIONS

Ulnar contact patterns were changed as a result of VV implant positioning using reverse-engineered DH implants, most notably on the medial aspect of the joint. Implant positioning plays a crucial role in producing contact patterns more like those observed in the native joint.

CLINICAL RELEVANCE

Recent clinical evidence reports nonsymmetrical ulnar wear after DHH. This work suggests that implant positioning is likely a contributing factor and that more exact implant positioning may lead to better clinical outcomes.

In vitro wear of ultrahigh-molecular-weight polyethylene and vitamin E blended highly cross-linked polyethylene in linked, semiconstrained total elbow replacement prostheses

BACKGROUND

The objectives of this study were to develop a clinically relevant in vitro elbow wear test and to compare the polyethylene wear of 2 total elbow replacement prostheses, one that uses conventional gamma-irradiated polyethylene (CPE) and one that uses vitamin E blended and cross-linked polyethylene (VE-HXPE) bushings.

MATERIALS AND METHODS

The test protocol applied 0° to 85° flexion-extension motions and imposed a constant 4.5° varus malalignment of the ulnar relative to the humeral implant under a variable joint load profile at a frequency of 1 Hz. The implants were tested for 3 million cycles (Mc) in a bovine serum lubricant at 37°C ± 3°C. Polyethylene wear was determined gravimetrically. Wear particles were isolated and characterized.

RESULTS

Clinically relevant polyethylene bushings wear mechanisms were observed. After 3 Mc, the mean CPE wear rate was 9.3 ± 2.8 mm³/Mc, significantly lower than that reported for hip and knee implants but comparable to that of ankle (7.4 ± 1.3 mm³/Mc) devices. The mean VE-HXPE wear rate was 0.8 ± 0.2 mm³/Mc, comparable to that of hip and knees devices. The mean equivalent circle diameter and aspect ratio were 0.17 ± 0.01 µm and 1.99 ± 0.18 for the CPE and 0.15 ± 0.02 µm and 1.81 ± 0.16 for the VE-HXPE particles.

CONCLUSION

The test replicated clinically observed failure modes for CPE devices. The use of VE-HXPE led to an order of magnitude reduction in polyethylene wear. Further clinical evaluation is necessary to determine if this translates into reduced complications of total elbow replacement associated with wear.

Radiocapitellar contact characteristics during prosthetic radial head subluxation

BACKGROUND

Metallic radial head prostheses are often used in the management of comminuted radial head fractures and elbow instability. We hypothesized that during radiocapitellar subluxation, the contact pressure characteristics of an anatomic radial head prosthesis will more closely mimic those of the native radial head compared with a monopolar circular or a bipolar circular radial head design.

MATERIALS AND METHODS

With use of 6 fresh frozen cadaver elbows, mean radiocapitellar contact pressures, contact areas, and peak pressures of the native radial head were assessed at 0, 2, 4, and 6 mm of posterior subluxation. These assessments were repeated after the native radial head was replaced with anatomic, monopolar circular and bipolar circular prostheses.

RESULTS

The joint contact pressures increased with the native and the prosthetic radial head subluxation. The mean contact pressures for the native radial head and anatomic prosthesis increased progressively and significantly from 0 to 6 mm of subluxation (native, 0.6 ± 0.0 MPa to 1.9 ± 0.2 MPa; anatomic, 0.7 ± 0.0 MPa to 2.1 ± 0.3 MPa; P < .0001). The contact pressures with the monopolar and bipolar prostheses were significantly higher at baseline and did not change significantly further with subluxation (monopolar, 2.0 ± 0.1 MPa to 2.2 ± 0.2 MPa [P = .31]; bipolar, 1.7 ± 0.1 MPa to 1.9 ± 0.1 MPa [P = .12]). The pattern of increase in contact pressures with the anatomic prosthesis mimicked that of the native radial head. Conversely, the circular prostheses started out with higher contact pressures that stayed elevated.

CONCLUSION

The articular surface design of a radial head prosthesis is an important determinant of joint contact pressures.

Elbow arthroplasty: where are we today? A narrative review

Background The elbow joint is a complex compound articulation, with a linking role within the upper limb kinematics. Its hinge function allows for proper placement of our working instrument (the hand) in the space surrounding us, directed by the shoulder joint. Both reliable mobility and stability are essential elements to allow for consistent bridging of the distance we aim to achieve in common daily activities. Sufficient flexion and extension are required to ensure both the patients' independence and the dignity. Next to the hinge, a radio-ulnar rotation with precise co-operation of forearm and wrist spin enhances the linking function with accurate precision instrument manipulation. Arthritis of the elbow joint or cubarthritis, whether primary or secondary, may not be as highly prevalent as hip or knee arthritis, but its impact on daily live certainly cannot be underestimated. Methods Current treatment options for failing cubarthritis are reviewed. Results Surgical techniques to reconstruct or replace the elbow joint are currently increasingly efficient with mounting long-term outcome reports. Debridement techniques including open or arthroscopic Outerbridge-Kashiwaghi procedure often delays joint replacement. Implants for joint arthroplasty focus on the ulna-humeral joint mostly with semi-constrained linked techniques, but there is a trend towards total joint replacement including the radiocapitellar joint. Conclusion In this independent review article, elbow joint failure due to cubarthritis and an overview of its current state-of-the-art orthopaedic treatment algorithm is presented, with its indications, advantages, risks and outcome.

Total elbow arthroplasty is moving forward: Review on past, present and future

The elbow joint is a complex joint, which, when impaired in function, leads to severe disability. In some cases however, an arthroplasty might be an appropriate treatment. In the past four decades, large steps have been taken to optimize this treatment in order to achieve better post-operative outcomes. To understand these progresses and to discover aspects for upcoming improvements, we present a review on the past developments, the present state of affairs and future developments which may improve patient care further.

The effect of prosthetic radial head geometry on the distribution and magnitude of radiocapitellar joint contact pressures

PURPOSE

To determine if radiocapitellar contact pressures would be elevated with nonanatomical (circular) prostheses over those mimicking native anatomy and if such pressures would be related to the depth and contour of the articular dish and to the pattern of prosthetic articulation against the lateral trochlear ridge.

METHODS

Three commercially available circular radial head designs were compared with an anatomical radial head and 2 modified anatomical prototype radial head designs in 10 cadaveric specimens. Each prosthesis and specimen combination was loaded in neutral rotation and maximal extension with a custom testing apparatus while measuring contact areas and pressures using thin-film pressure sensors.

RESULTS

Anatomical radial head prototype 2 had similar radiocapitellar contact areas and mean pressures as the native radial head; all other designs showed significant decreases in contact area and increased mean pressures. Peak contact pressures were also measured and were significantly elevated with all prostheses tested. Anatomical designs are statistically more likely to mimic normal contact with the lateral trochlear ridge and its adjacent sulcus than circular prostheses. They are also significantly less likely to have contact pressures above the 5 MPa threshold that is thought to be harmful to cartilage. The depth of the articular dish had a significant effect on contact area and pressure.

CONCLUSIONS

Commercially available radial head prostheses demonstrated reduced radiocapitellar contact areas and elevated contact pressures during compressive loading. These were significantly greater with symmetrical circular prostheses than with asymmetrical elliptical designs. The prosthesis that best mimicked native contact behavior was the anatomical radial head prototype 2 owing to its design for articulating with the capitellum, the lateral trochlear ridge, and the sulcus between.

CLINICAL RELEVANCE

Because radial head prostheses have the potential to cause capitellar erosion or arthritic change, those with lower contact pressures may lead to fewer such complications.

Influence of radial head prosthetic design on radiocapitellar joint contact mechanics

HYPOTHESIS

Our aim was to test whether anatomically designed metallic radial head implants could better reproduce native radiocapitellar contact pressure and areas than nonanatomic implants.

METHODS

The distal humerus and proximal radius from 6 cadaveric upper extremities were serially tested in supination with 100 N of compression force at 4 angles of flexion (0°, 30°, 60°, and 90°). By use of a thin flexible pressure transducer, contact pressures and areas were measured for the native radial head, an anatomic implant, a nonanatomic circular monopolar implant, and a bipolar nonanatomic implant. The data (mean contact pressure and mean contact area) were modeled using a 2-factor repeated-measures analysis of variance with P ≤ .05 considered to be significant.

RESULTS

The mean contact areas for the prosthetic radial heads were significantly less than those seen with the intact radial heads at every angle tested (P < .01). The mean contact pressures increased significantly with all prosthetic radial head types as compared with the native head. The mean contact pressures increased by 29% with the anatomic prosthesis, 230% with the monopolar prosthesis, and 220% with the bipolar prosthesis. Peak pressures of more than 5 MPa were more commonly observed with both the monopolar and bipolar prostheses than with the anatomic or native radial heads.

CONCLUSIONS

The geometry of radial head implants strongly influences their contact characteristics. In a direct radius-to-capitellum axial loading experiment, an anatomically designed radial head prosthesis had lower and more evenly distributed contact pressures than the nonanatomic implants that were tested.