Trapezium Bone Density-A Comparison of Measurements by DXA and CT

Superior bone fixation of conical compared with hemispherical trapezial cup design: an experimental radiostereometry study

PURPOSE

The most used cup designs for trapeziometacarpal (TMC) arthroplasty are of hemispherical and conical geometrical shape. Using a validated pig bone model, we compared the bone fixation using radiostereometry (RSA).

METHODS

Twenty saddle-shaped pig forefoot bones were prepared with insertion of bone markers and reaming. Hemispherical Type T cups (Beznoska, Kladno, Czech Republic) (N = 10) and conical Moovis cups (Stryker, Pusignan, France) (N = 10) of 9-mm diameter were inserted press-fit. The bones were fixed in cement blocks for stability, and the cups were loaded in a motorized test stand. First, a low-pressure cyclic load test (0-150N) with 130 compression cycles was performed. Next, a push-in test of progressive loads with 50N increments (range: 150-900N) was applied until a visual change in cup position appeared. Cup migration was evaluated with RSA after every new load application. Cup failure was defined as total translation > 0.5 mm between two load applications.

RESULTS

Both cup types tolerated a compression load of 450 N without failure. Beyond this load level, the total translation cup migration of mean 0.20 mm (95% CI 0.11; 0.30) for the Type T group was higher than mean 0.10 mm (95% CI 0.06; 0.15) of the Moovis group (p = 0.046). The Hazard ratio for failure was 0.52 (95% CI 0.12; 2.17) (p = 0.37), indicating that the risk of failure was two-fold higher in the Type T group.

CONCLUSION

We conclude that conical TMC cups have superior fixation as compared to hemispherical cups above a loading level of 450 N, which correspond to a 3.8 kg tip-pinch. In a clinical perspective, based on the fixation strength of both cup types, it seems safe to allow light-load activities of daily living such as buttoning a shirt and using a key shortly after surgery and until sufficient osseointegration is achieved.

Comparison of two radiographic landmarks for centering the trapezial component in total trapeziometacarpal arthroplasty

We performed a retrospective review of standard anteroposterior and lateral radiographs of the thumb in 80 patients, to compare two radiographic landmarks, in terms of mediolateral bone support, for centering the trapezial component in total joint arthroplasty. On anteroposterior view, we identified the distal articular surface of the trapezium and trapezium width, and defined the two midpoints as radiographic landmarks for positioning a 9-mm trapezial cup. Mean trapezium width was significantly greater than the distal articular surface of the trapezium, and the midpoints did not match. Thus, after positioning simulated 9-mm prosthetic cups centered on each landmark, the residual radial bone distance was significantly greater using the landmark based on trapezium width. The mean value was 33% greater with this landmark, and the minimum value was 2.1 mm, compared to 0.2 mm using the landmark based on the distal articular surface. Our study thus suggested that the midpoint of the trapezium width is the more relevant radiographic landmark for centering the trapezial prosthetic cup in total joint arthroplasty, by preserving better bone stock on the radial side without depleting the ulnar side. On an intraoperative anteroposterior fluoroscopic view, this landmark could be used to check cup positioning.

The mechanical fixation of a cementless conical cup in cortical versus cancellous trapezial bone: an experimental study

In a pig bone model mimicking the human trapezium, we assessed the cementless fixation of trapezial cups in cancellous and cortical bone. Thirty-two saddle-shaped pig forefoot bones were prepared for cup fixation. Conical Konos cups (Beznoska, Kladno, Czech Republic) size 9 mm or 10 mm diameter were impacted. Cup migration was evaluated with repeated radiostereometry after intervals of cyclic-load tests. Migration increased for every load-interval up to 750 N and was higher with cancellous bone fixation than with cortical bone fixation. In cancellous bone, 9 mm cups migrated more than 10 mm cups. At the highest load (1050 N), the cumulative implant survival was 88% for 10 mm cups with cortical bone fixation and 13% for 9 mm cups with cancellous bone fixation. We conclude that mechanical fixation of conical Konos cups was better in cortical than in cancellous bone. Our results further suggest that the largest possible cup diameter should be used.

Use of Bone Health Evaluation in Orthopedic Surgery: 2019 ISCD Official Position

This position development conference (PDC) Task Force examined the assessment of bone status in orthopedic surgery patients. Key questions included which orthopedic surgery patients should be evaluated for poor bone health prior to surgery and which subsets of patients are at high risk for poor bone health and adverse outcomes. Second, the reliability and validity of using bone densitometry techniques and measurement of specific geometries around the hip and knee before and after arthroplasty was determined. Finally, the use of computed tomography (CT) attenuation coefficients (Hounsfield units) to estimate bone quality at anatomic locations where orthopedic surgery is performed including femur, tibia, shoulder, wrist, and ankle were reviewed. The literature review identified 665 articles of which 198 met inclusion exclusion criteria and were selected based on reporting of methodology, reliability, or validity results. We recommend that the orthopedic surgeon be aware of established ISCD guidelines for determining who should have additional screening for osteoporosis. Patients with inflammatory arthritis, chronic corticosteroid use, chronic renal disease, and those with history of fracture after age 50 are at high risk of osteoporosis and adverse events from surgery and should have dual energy X-ray absorptiometry (DXA) screening before surgery. In addition to standard DXA, bone mineral density (BMD) measurement along the femur and proximal tibia is reliable and valid around implants and can provide valuable information regarding bone remodeling and identification of loosening. Attention to positioning, selection of regions of interest, and use of special techniques and software is required. Plain radiographs and CT provide simple, reliable methods to classify the shape of the proximal femur and to predict osteoporosis; these include the Dorr Classification, Cortical Index, and critical thickness. Correlation of these indices to central BMD is moderate to good. Many patients undergoing orthopedic surgery have had preoperative CT which can be utilized to assess regional quality of bone. The simplest method available on most picture archiving and communications systems is to simply measure a regions of interest and determine the mean Hounsfield units. This method has excellent reliability throughout the skeleton and has moderate correlation to DXA based on BMD. The prediction of outcome and correlation to mechanical strength of fixation of a screw or implant is unknown.