Stem Length and Neck Resection on Fixation Strength of Press-Fit Radial Head Prosthesis: An In Vitro Model.

PURPOSE: Various radial head prosthesis designs are currently in use. Few studies compare different prosthetic designs. We hypothesized that increasing a cementless implant stem's length would reduce stem-bone micromotion, with both short and long neck cuts. We also hypothesized that a minimum stem length might be required for the initial fixation strength of a press-fit implant. METHODS: In 16 fresh-frozen cadaveric elbows (8 pairs), the radial head and neck were cut either 10 or 21 mm below the top of the head. Modular cementless stems were inserted and sequentially lengthened in 5-mm increments. Micromotion under eccentric loading was tested after each incremental change. RESULTS: Incremental lengthening of the prosthetic stem and the amount of neck resection (10-mm cut vs 21-mm cut) both had a significant effect on micromotion. After a 10-mm radial head-neck resection, we observed a significant decrease in micromotion with stem lengths of 25 mm or greater, whereas with 21 mm of neck resection there was no further reduction in micromotion with increased stem length. These differences can be explained, at least in part, by the concept of the cantilever quotient: the ratio of the head-neck length outside the bone to the total length of the implant. CONCLUSIONS: The length of the stem affects the initial stability of press-fit radial head prostheses when the level of head and neck resection is at the minimum (ie, 10 mm) for currently available prosthetic designs. At this resection level, stems 25 mm or greater had significantly higher initial stability, but all stem lengths tested had mean micromotion values within the threshold for bone ingrowth. CLINICAL RELEVANCE: The length of a radial head prosthetic stem affects the initial stability of press-fit radial head prostheses when the level of head and neck resection is at the minimum (ie, 10 mm) for currently available prosthetic designs.

Intraobserver and interobserver reliability of the modified Walch classification using radiographs and computed tomography.

BACKGROUND: The Walch classification was introduced to classify glenoid morphology in primary glenohumeral osteoarthritis. A modified Walch classification was recently proposed, with 2 additional categories, B3 (monoconcave glenoid with posterior bone loss leading to retroversion > 15° or subluxation > 70%) and D (excessive anterior subluxation), as well as a more precise definition of subtypes A2 and C. The purpose of this study was to evaluate the intraobserver and interobserver agreement of the modified Walch classification system using both plain radiographs and computed tomography (CT). METHODS: Three fellowship-trained shoulder surgeons blindly and independently evaluated radiographs and CT scans of 100 consecutive shoulders (98 patients) with primary glenohumeral osteoarthritis and classified all shoulders according to the modified Walch classification in 4 separate sessions, each 4 weeks apart. Statistical analysis with the κ coefficient was used to evaluate reliability. RESULTS: The first reading by the most senior observer on the basis of CT scans was used as the gold standard (distribution: A1, 18; A2, 12; B1, 20; B2, 25; B3, 22; C, 1; and D, 2). The average intraobserver agreement for radiographs and CT scans was 0.73 (substantial; 0.72, 0.74, and 0.72) and 0.73 (substantial; 0.77, 0.69, and 0.72), respectively. The average interobserver agreement was 0.55 (moderate; 0.61, 0.51, and 0.53) for radiographs and 0.52 (moderate; 0.63, 0.50, and 0.43) for CT scans. CONCLUSION: Intraobserver agreement of the modified Walch classification was substantial both for axillary radiographs and for CT scans. Interobserver agreement was fair. Although the modified Walch classification represents an improvement over the original classification, automated computer-based analysis of CT scans may be needed to further improve the value of this classification.

Management of Failed Rotator Cuff Repair in Young Patients.

Management of failed rotator cuff repair may be very difficult, especially in young patients. Various nonmodifiable and modifiable patient factors, including age, tendon quality, rotator cuff tear characteristics, acute or chronic rotator cuff tear, bone quality, tobacco use, and medications, affect rotator cuff repair healing. Surgical variables, such as the technique, timing, tension on the repair, the biomechanical construct, and fixation, as well as the postoperative rehabilitation strategy also affect rotator cuff repair healing. Variable outcomes have been reported in patients who undergo revision rotator cuff repair; however, a systematic surgical approach may increase the likelihood of a successful outcome. Numerous cellular and mechanical biologic augments, including platelet-rich plasma, platelet-rich fibrin matrix, mesenchymal stem cells, and acellular dermal matrix grafts, have been used in rotator cuff repair; however, conflicting or inconclusive outcomes have been reported in patients who undergo revision rotator cuff repair with the use of these augments. A variety of tendon transfer options, including latissimus dorsi, teres major, lower trapezius, pectoralis minor, pectoralis major, combined pectoralis major and latissimus dorsi, and combined latissimus dorsi and teres major, are available for the management of massive irreparable rotator cuff tears. Ultimately, the optimization of surgical techniques and the use of appropriate biologic/tendon transfer techniques, if indicated, is the best method for the management of failed rotator cuff repair.

The Posterior Bundle's Effect on Posteromedial Elbow Instability After a Transverse Coronoid Fracture: A Biomechanical Study.

PURPOSE: There has been increased interest in the role of the posterior bundle of the medial collateral ligament (pMUCL) in the elbow, particularly its effects on posteromedial rotatory stability. The ligament's effect in the context of an unfixable coronoid fracture has not been the focus of any study. The purposes of this biomechanical study were to evaluate the stabilizing effect of the pMUCL with a transverse coronoid fracture and to assess the effect of graft reconstruction of the ligament. METHODS: We simulated a varus and internal rotatory subluxation in 7 cadaveric elbows at 30°, 60°, and 90° elbow flexion. The amount of ulnar rotation and medial ulnohumeral joint gapping were assessed in the intact elbow after we created a transverse coronoid injury, after we divided the pMUCL, and finally, after we performed a graft reconstruction of the pMUCL. RESULTS: At all angles tested, some stability was lost after cutting the pMUCL once the coronoid had been injured, because mean proximal ulnohumeral joint gapping increased afterward by 2.1, 2.2, and 1.3 mm at 90°, 60°, and 30°, respectively. Ulnar internal rotation significantly increased after pMUCL transection at 90°. At 60° and 30° elbow flexion, ulnar rotation increased after resection of the coronoid but not after pMUCL resection. CONCLUSIONS: An uninjured pMUCL stabilizes against varus internal rotatory instability in the setting of a transverse coronoid fracture at higher flexion angles. Further research is needed to optimize graft reconstruction of the pMUCL. CLINICAL RELEVANCE: The pMUCL is an important secondary stabilizer against posteromedial instability in the coronoid-deficient elbow. In the setting of an unfixable coronoid fracture, the surgeon should examine for posteromedial instability and consider addressing the pMUCL surgically.

Validation of a Simple Overlay Device to Assess Radial Head Implant Length.

PURPOSE: A simple overlay device (SOD) was developed to measure radial head implant length. The purpose of this study was to determine the accuracy and reliability of this device for measuring experimental radial head implant length. METHODS: Five fresh frozen cadavers were implanted with sequentially longer implants, adjusted by neck length (0, 2, 4, and 8 mm). Fluoroscopic images were obtained in 4 forearm positions: anteroposterior in supination in full extension, anteroposterior in pronation in full extension, supinated in 45° of flexion, and neutral in 45° of flexion. The SOD measurements (made by 2 observers) were compared with the native original radial head (control) to assess implant length. In addition, gapping of the ulnohumeral joint space was measured for comparison purposes. RESULTS: The measured radial head and neck lengths for the specimens were 33, 39, 31, 34, and 42 mm. The difference between the actual radial head and neck lengths and those measured with the SOD template averaged less than 2 mm for all 4 collar sizes, except in 1 measurement in which the bicipital tuberosity could not be visualized. The median intraclass correlation coefficients for observer 1 compared with the SOD were 0.94 to 0.99. The median intraclass correlation coefficients between observers were 0.88 to 0.95. For both observers, elbow position, collar height, and the 2 variables combined did not significantly affect the SOD values. The other method that was evaluated, that of measurement of the ulnohumeral joint space, had higher interobserver variability versus the SOD, and allowed detection of lengthening of over 4 mm. CONCLUSIONS: The SOD is a reliable method for simply assessing radial head length with radiographs and can accurately detect 2 mm or more of proximal radial lengthening. CLINICAL RELEVANCE: The SOD is a simple and accurate method that can help to optimize radial head sizing.

Anthropometric Study of the Radiocapitellar Joint.

PURPOSE: There is scant knowledge about the relationship between the size of the radial head and the size of the capitellum. Also, no data exist comparing the size of the capitellum between the left and the right elbow. METHODS: Eight pairs of elbows and 12 single elbows from fresh-frozen cadavers were obtained for this study. The vertical height and anterior width of the capitellum were measured with digital calipers. Four different measurements were performed at the radial head: longest outer diameter, shortest outer diameter, the long dish diameter, and short dish diameter. The Pearson intrarater intraclass correlation coefficients were obtained for all measurements. RESULTS: For the paired elbows, the correlations ranged between 0.95 and 0.96 for the capitellar dimensions and 0.77 and 0.98 for the radial head dimensions. The correlations between the long outer diameter of the radial head with the vertical height and the anterior width of the capitellum were 0.8 and 0.9, respectively. CONCLUSIONS: There is a high correlation between the long outer diameter of the radial head and the vertical height of the capitellum as well its anterior width. There is also a high correlation between the left and the right elbow. CLINICAL RELEVANCE: These findings are relevant to radiocapitellar arthroplasty and may be useful for radiocapitellar prosthetic design as well as in the preoperative planning of cases in which the radial head and/or the capitellum is destroyed.

The effect of a radial neck notch on press-fit stem stability: a biomechanical study on 7 cadavers.

BACKGROUND: Minimal micromotion is necessary for osteointegration of cementless radial head prostheses. When radial head fractures extend longitudinally, where the neck cut for prosthetic replacement should be made is uncertain. We hypothesized that complete resection of the notched portion of a radial neck confers no advantage in initial stability compared with not resecting the defect and inserting the implant into a notched radial neck. MATERIALS AND METHODS: The radii of 7 cadavers underwent radial head resection and implantation with a 25-mm-long press-fit radial head stem. Before implantation, a 5-mm-long notch that was less than 1-mm wide was made in the radial neck. After the stem-bone micromotion was recorded, the proximal 5 mm of radial neck, incorporating the entire notch, was cut away, the stem was inserted 5 mm further, and the resulting micromotion was recorded. RESULTS: The mean micromotion measured in the presence of a cortical notch was 51 ± 6 µm. After the neck was circumferentially cut and the stem was advanced, the micromotion (46 ± 9 µm) was not statistically significantly different. DISCUSSION: Initial stability of an adequately sized cementless stem in the presence of a 5-mm-long cortical notch was well within the threshold needed for bone ingrowth (<100 µm). In addition, there was no reduction of micromotion after the notch-containing portion of the radial neck was resected and the stem was advanced. Making a neck cut distal to a 5-mm-long, 1-mm-wide cortical notch does not confer added stability. Thus, surgeons can preserve bone stock and avoid an aggressive neck cut.

Varus posteromedial rotatory instability: a biomechanical analysis of posterior bundle of the medial ulnar collateral ligament reconstruction.

BACKGROUND: Recently, there has been growing interest in the involvement of the posterior bundle of the medial ulnar collateral ligament (pMUCL) in varus posteromedial rotatory instability (PMRI). Varus PMRI has been observed clinically, but the degree of involvement of the pMUCL remains unclear. This study assessed the degree to which the pMUCL is involved in stabilizing the elbow and the feasibility of a pMUCL reconstruction to restore stability. METHODS: Movements simulating PMRI were performed in 8 cadaveric elbows. Joint gapping values were obtained by 3-dimensional motion capture for the proximal and distal aspects of the ulnohumeral joint. Specimens were assessed at "intact," "cut coronoid + pMUCL," "reconstruction," and "cut anterior aspect MUCL + reconstruction" conditions with mechanical testing at 30°, 60°, and 90° of elbow flexion. RESULTS: Proximal joint gapping significantly increased from intact to cut coronoid + pMUCL at 60° and 90°, and distal joint gapping significantly increased at 90°. In the reconstruction condition, joint gapping across the proximal joint at 60° and 90° significantly recovered, as did distal joint gapping at 90°. In the cut anterior aspect MUCL + reconstruction condition, no significant increase occurred in proximal or distal joint gapping. CONCLUSIONS: Transection of the pMUCL with a coronoid fracture leads to increased joint gapping, suggesting the presence of PMRI. PMRI can still occur with an intact lateral ligamentous complex. A pMUCL tendon graft reconstruction confers some elbow stability in this injury mechanism.

Isolated ligamentous injury can cause posteromedial elbow instability: a cadaveric study.

BACKGROUND: Elbow posteromedial rotatory instability (PMRI) is known to occur with fracture of the anteromedial coronoid and injury to the posterior bundle of the medial ulnar collateral ligament (pMUCL). However, whether instability results from isolated pMUCL injury remains unclear. The purpose of this study was to quantify displacement about the ulnohumeral joint to evaluate whether isolated sectioning of the pMUCL results in elbow PMRI. METHODS: Nine cadaveric elbows underwent movements simulating PMRI by application of axial compression with varus and internal rotation moments. Gapping values at both the proximal and distal aspects of the medial ulnohumeral joint were then recorded for "intact" and "pMUCL-sectioned" elbows at positions of 30°, 60°, and 90° of flexion. RESULTS: After pMUCL transection, torsion increased by 2.6° ± 0.7° (P = .054) at 30° and 4.5° ± 1.2° (P = .039) at 60° of flexion. Proximal ulnohumeral joint gapping also increased at 30° (1.4 ± 0.4 mm; P = .039), 60° (1.5 ± 0.6 mm; P = .039), and 90° (1.5 ± 0.7 mm; P = .017), respectively. No increases in distal ulnohumeral gapping occurred at any angle of flexion. DISCUSSION: Sectioning of the pMUCL results in significant increases in torsion and displacement about the proximal ulnohumeral joint. Our findings demonstrate that elbow PMRI can occur secondary to isolated ligamentous injury. Clinicians mindful of this previously unrecognized role of the pMUCL as a stabilizer may wish to consider methods of restoring pMUCL integrity when treating medial elbow instability.

Importance of the posterior bundle of the medial ulnar collateral ligament.

BACKGROUND: There has been a renewed interest in the pathomechanics of elbow dislocation, with recent literature having suggested that the medial ulnar collateral ligament is more often disrupted in dislocations than the lateral ligamentous complex. The purpose of this serial sectioning study was to determine the influence of the posterior bundle of the medial ulnar collateral ligament (pMUCL) as a stabilizer against elbow dislocation. METHODS: An elbow dislocation was simulated in 5 cadaveric elbows by mechanically applying an external rotation moment and valgus force. Medial ulnohumeral joint gapping was measured at 30°, 60°, and 90° of flexion in an intact elbow after sectioning of the medial collateral ligament's anterior bundle (aMUCL) and then after sectioning of the pMUCL as well. RESULTS: After sectioning of the aMUCL, the pMUCL was able to stabilize the joint against dislocation. After aMUCL sectioning, the proximal joint space significantly increased by 4.2 ± 0.6 mm at 30° of flexion and 2.6 ± 0.3 mm at 60° of flexion, although it did not dislocate. The gapping increase of 0.9 ± 0.6 at 90° of flexion did not reach significance. After sectioning of the pMUCL (after having already sectioned the aMUCL), all of the specimens frankly dislocated at all flexion angles. CONCLUSIONS: An intact pMUCL can prevent elbow dislocation and limited joint subluxation to within 6.6 mm. Our findings indicate that repair or reconstruction may be warranted in certain circumstances (ie, residual instability after operative management of a terrible triad injury or after aMUCL reconstruction).

Hemiarthroplasty versus reverse shoulder arthroplasty for treatment of proximal humeral fractures: a meta-analysis.

BACKGROUND: We performed a meta-analysis of studies with at least Level IV evidence to compare outcomes between hemiarthroplasty and reverse shoulder arthroplasty for the treatment of proximal humeral fractures. METHODS: Three electronic databases (PubMed, Cochrane, and EMBASE) were searched. The quality of each study was investigated, and data on radiographic and functional outcomes were extracted and analyzed. RESULTS: The analysis included 1 Level I study, 1 Level II study, 3 Level III studies, and 2 Level IV studies. Reverse shoulder arthroplasty was more favorable than hemiarthroplasty in forward elevation (P < .001), abduction (P < .001), tuberosity healing (P = .002), Constant score (P < .001), American Shoulder and Elbow Surgeons score (P < .001), and Disabilities of the Arm, Shoulder and Hand score (P = .001). Only external rotation (P = .85) was not in favor of reverse shoulder arthroplasty. CONCLUSIONS: The available literature suggests that reverse shoulder arthroplasty performed to address complex proximal humeral fractures might result in more favorable clinical outcomes than hemiarthroplasty performed for the same indication.

The effect of capitellar impaction fractures on radiocapitellar stability.

PURPOSE: To determine the effect of capitellar impaction fractures on radiocapitellar stability in a model that simulated a terrible triad injury. METHODS: Six cadaveric elbows were dissected free of skin and muscles. Tendons were preserved. The lateral collateral ligament was released and repaired (surgical control). Two sizes of capitellar impaction defects were created. After lateral collateral ligament release and repair, we then sequentially created osseous components of a terrible triad injury (partial radial head resection and coronoid fracture) through an olecranon osteotomy that was fixed with a plate. Radiocapitellar stability was recorded after the creation of each new condition. RESULTS: Significantly less force was required for radiocapitellar subluxation after the creation of 20° and 40° capitellar defects compared with the surgical control (intact capitellum). After the addition of a Mason type II radial head defect and then a coronoid defect, stability decreased significantly further. CONCLUSIONS: Impaction fractures of the distal portion of the capitellum may contribute to a loss of radiocapitellar stability, particularly in an elbow fracture-dislocation. CLINICAL RELEVANCE: Because these injuries may be unrecognized, consideration should be given to diagnosing and addressing them.

Heterotopic ossification formation after fracture-dislocations of the elbow.

BACKGROUND: Heterotopic ossification (HO) is a serious complication of traumatic elbow injuries, particularly fracture-dislocations. Limited data exist in the literature regarding the risk factors associated with HO formation in these injuries. The purpose of this study was to review the incidence of HO after fracture-dislocation of the elbow and to identify potential risk factors associated with its formation. METHODS: Twenty-seven patients (28 elbows) were surgically treated for elbow fracture-dislocations during 8 years, with an average follow-up of 14 months. Records were reviewed with attention paid to several factors: demographic data, comorbidities, time interval from injury to surgical intervention, number of closed reductions attempted before surgery, surgical approach, management of the radial head, treatment of the anterior capsular injury, and coronoid fixation. RESULTS: Of the 28 elbows, 12 (43%) developed HO postoperatively; 9 of 28 elbows underwent multiple attempted closed reductions before definitive surgical stabilization, with HO formation in 7 of the 9 (77%). Time to surgery, age, gender, radial head fixation or replacement, coronoid open reduction and internal fixation, capsular repair, and medical comorbidities were not found to influence HO formation, although the performance of multiple reductions was identified as a risk factor. DISCUSSION: HO developed in 77% of patients with multiple attempted closed reductions. We found a 43% incidence of HO in patients surgically treated for elbow fracture-dislocations. Neither time to surgery after the injury nor demographic or other factors relating to the manner in which associated osseous or soft tissue injuries were managed influenced the formation of HO.

Outcomes of arthroscopic treatment of osteochondritis dissecans of the capitellum and description of the technique.

HYPOTHESIS: We hypothesize that a technique for all-arthroscopic fixation of capitellum osteochondritis dissecans (OCD) lesions using suture fixation and autogenous iliac crest bone grafting offers a successful alternative to open internal fixation techniques as shown by 2-year validated patient-reported outcomes. METHODS: Our technique uses arthroscopic all-inside suture fixation with iliac crest autogenous bone grafting. The procedure was performed on 4 elite-level, adolescent athletes presenting with 5 unstable capitellum OCD lesions resulting in elbow pain, limited range of motion, and decreased ability to play. Magnetic resonance imaging showed an unstable OCD lesion, which was correlated with arthroscopy. Postoperatively, patients were evaluated by the short version of the Disabilities of the Arm, Shoulder and Hand questionnaire; Oxford Elbow and Mayo Elbow scores; visual analog scale; postoperative range of motion; and return to play. RESULTS: Three female patients and one male patient aged 13 to 15 years underwent the procedure. The mean final follow-up period was 2.8 years. Union was achieved in all patients, as seen on magnetic resonance imaging at a mean of 3 months. At follow-up, the mean loss of extension was 2°. Mean flexion was 153°. There was no loss of supination or pronation. The mean score on the short version of the Disabilities of the Arm, Shoulder and Hand questionnaire was 11. The mean Mayo Elbow score was 88. The mean Oxford Elbow score was 42. The mean visual analog scale score was 2. The mean time to return to play was 4 months. All patients continued to compete at an elite level. There were no infections or cases of fixation failure, and no patients required conversion to open surgery or needed revision surgery. CONCLUSION: Arthroscopic all-inside fixation of unstable OCD lesions is a successful technique, facilitating athletes to return to an elite level of play.

Canal preparation for prosthetic radial head replacement: rasping versus reaming.

BACKGROUND: While many design-specific features of radial head prostheses have been studied (ie, geometry and surface coating), the optimum technique for canal preparation has not been determined. We hypothesized that preparation of the radial canal with a reamer would allow for the accommodation of a larger stem diameter versus following canal preparation with a rasp, and would provide acceptable stem-bone micromotion. METHODS: Paired proximal radii from 7 cadavers were prepared by a rasp on one side and a reamer on the contralateral side. Cementless radial head stems of increasing diameter were sequentially implanted up to the maximum size or until a fracture occurred and the micromotion between the stem and bone was recorded. RESULTS: In 3 of 5 pairs, at least a 1 mm larger stem size fit into the canal after reaming versus after rasping (P = .04). 5 of 7 radii fractured secondary to intentional stem oversizing. For the optimally-sized stems, similar micromotion values were observed whether the canal was rasped (41 ± 6 µm) or reamed (44 ± 6 µm) (P = .72). DISCUSSION: This study investigated an aspect of radial head arthroplasty technique about which little has currently been published. It is possible that use of a reamer rather than a rasp, while providing similar initial stability, might expand the stem size options for initial press-fit stability, and decrease the risk of fracture. CONCLUSION: Radial canal preparation with a reamer allows for implantation of a 1 mm larger stem diameter versus rasping, while providing comparable initial stability to that achieved after rasping.

Stress shielding around radial head prostheses.

PURPOSE: Stress shielding is known to occur around rigidly fixed implants. We hypothesized that stress shielding around radial head prostheses is common but nonprogressive. In this study, we present a classification scheme to support our radiographic observations. METHODS: We reviewed charts and radiographs of 86 cases from 79 patients with radial head implants from both primary and revision surgeries between 1999 and 2009. Exclusion criteria included infection, loosening, or follow-up of less than 12 months. We classified stress shielding as: I, cortical thinning; II, partially (IIa) or circumferentially (IIb) exposed stem; and III, impending mechanical failure. RESULTS: Of 26 well-fixed stems, 17 (63%) demonstrated stress shielding: I = 2, II = 15 (IIa = 12, IIb = 3), and III = 0. We saw stress shielding with all stem types: cemented or noncemented; long or short; and straight, curved, or tapered. The only significant difference was that stems implanted into the radial shaft had less stress shielding than stems implanted into the neck or tuberosity (P = .03). The average follow-up was 33 months (range, 13-70 mo). Stress shielding was detectable by an average of 11 months (range, 1-15 mo). The pattern of bone loss was similar in 16 of 17 cases (94%), starting on the outer periosteal cortex. The 3 cases with circumferential exposure of the stem (stage IIb) averaged 2.6 mm (range, 1-4 mm) of exposed stem. Stress shielding never extended to the bicipital tuberosity, and there were no cases of impending mechanical failure. CONCLUSIONS: Stress shielding around radial head prostheses is common, regardless of stem design. However, it is typically minor, nonprogressive, and of questionable clinical consequence. TYPE OF STUDY/LEVEL OF EVIDENCE: Therapeutic IV.

Effect of radial head malunion on radiocapitellar stability.

BACKGROUND: Management for Mason type II radial head fractures is controversial. We hypothesized that angulation or depression of a marginal radial head fragment would affect radiocapitellar stability similarly to fragment excision. MATERIALS AND METHODS: A Mason type II radial head fracture was created in 6 cadaveric elbows by excising a segment from the anterolateral quadrant that was 30% of the diameter of the articular surface. Radiocapitellar stability was recorded under 5 sets of conditions: (1) intact radial head (intact), (2) 30% surface area fragment resected (partially excised), (3) anatomic fragment fixation with screws (fixed), (4) fragment fixation with 2 mm of depression relative to the articular surface (depressed), and (5) fragment fixation after a 30° wedge resection (angulated). RESULTS: The forces required to subluxate the joint were greatly reduced after fragment excision (5 ± 1 N; P = .0001) and restored to normal (21 ± 1 N; P = .9) after anatomic fixation of the excised fragment. The peak forces were significantly reduced with fragment depression (4 ± 1 N) and angulation (4 ± 2 N; P = .0001). CONCLUSION: A radial head fracture that is depressed 2 mm or angulated 30° may cause up to an 80% loss of concavity-compression stability of the radiocapitellar joint.

Effect of stem length on prosthetic radial head micromotion.

BACKGROUND: Osteointegration of press-fit radial head implants is achieved by limiting micromotion between the stem and bone. Aspects of stem design that contribute to the enhancement of initial stability (ie, stem diameter and surface coating) have been investigated. The importance of total prosthesis length and level of the neck cut has not been examined. METHODS: Cadaveric radii were implanted with cementless, porous-coated radial head stems. We resected 10, 12, 15, 20, and 25 mm of radial neck in each specimen. Stem-bone micromotion was measured after each cut. Values were expressed in terms of quotients (cantilever quotient). RESULTS: A threshold effect was observed at 15 mm of neck resection (cantilever quotient, 0.4), with a significant increase in micromotion observed between 12 mm (40 ± 10 µm) and 15 mm (80 ± 25 µm). A cantilever quotient of 0.35 or less predicted implant stability, whereas implants with a cantilever quotient of 0.6 or more were unstable. In between, the stems were "at risk" of instability. CONCLUSION: Initial stem stability of a porous-coated, cementless radial head implant is dependent on length of the implant stem within bone and the level of the cut (amount of bone resected). Stability may be compromised by an implant with a combined head and neck length that is too long compared with the stem length within the canal. We found a critical ratio of exposed prosthesis to total implant length (cantilever quotient of 0.4), which puts the prosthesis at risk of inadequate initial stability. These data carry important implications for implant design and use.

Effect of hoop stress fracture on micromotion of textured ingrowth stems for radial head replacement.

BACKGROUND: Successful bone ingrowth around cementless implants requires adequate initial stability. Hoop stress fractures during stem insertion can potentially hinder prosthesis stability. HYPOTHESIS: We hypothesized that an oversized radial head prosthetic stem (1 mm "too large" and causing a hoop stress fracture during insertion) would result in an unacceptable amount of micromotion. MATERIALS AND METHODS: Grit-blasted radial head prosthetic stems were implanted into cadaveric radii. Rasp and stem insertion energies were measured, along with micromotion at the stem tip. The sizes were increased until a fracture developed in the radial neck. RESULTS: Prosthetic radial head stems that were oversized by 1 mm caused small cracks in the radial neck. Micromotion of oversized stems (42 ± 7 µm) was within the threshold conducive for bone ingrowth (<100 µm) and not significantly different from that for the maximum sized stems (50 ± 12 µm) (P ≥ .4). DISCUSSION: Contrary to our hypothesis, hoop stress fractures caused by implantation of a stem oversized by 1 mm did not result in loss of stability. Stem micromotion remained within the range for bone ingrowth and was not significantly diminished after the fracture. This suggests that if a crack occurs during the final stages of stem insertion, it may be acceptable to leave the stem in place without adding a cerclage wire. CONCLUSION: A small radial neck fracture occurring during insertion of a radial head prosthetic stem oversized by 1 mm does not necessarily compromise initial stability.

Effects of rasp mismatch on plasma spray radial head stems.

BACKGROUND: Radial head prosthetic stems designed for bone ingrowth are available with both plasma spray and grit blasted surfaces. A recent study comparing micromotion between the 2 demonstrated greater micromotion in the plasma spray than grit blasted stems, even though the latter had lower surface roughness. This raised the question that perhaps the size mismatch for grit-blasted radial head stems (0.5 mm) might be inadequate for plasma spray stems. HYPOTHESIS: A tighter initial press-fit with plasma spray radial head stems may be gained by preparation with an undersized rasp. METHODS: Paired cadaveric radii were implanted with plasma spray stems. The surgical control was prepared with a rasp designated for its corresponding stem size ("size-matched"), while the experimental group was prepared with a rasp 0.5 mm smaller than designated ("undersized"). RESULTS: The micromotion for the undersized rasp group (46 ± 12 µm) was not significantly different than for the size-matched rasp group (21 ± 12 µm) (P = .1). DISCUSSION: Contrary to our hypothesis, no reduction in micromotion was observed when using an undersized rasp with a plasma spray stem. The micromotion results were not different from those observed when using a size-matched rasp, and actually approached significance in the opposite direction. This may be due to the rough stem surface chipping away bone fragments, rather than the bone being cut away precisely as is done with a rasp. CONCLUSION: The use of an undersized rasp prior to implantation of a plasma spray radial head prosthesis does not confer any added benefit in terms of initial stability.