Purpose: According to the Finnish registry, the ABG1 stem has a 94% survival (90–98) at seven years. A multi-centric study in Torino found 99% survival at five years. These studies have however reported worrisome radiographic images showing absorption of the calcar in 27% of the cases. We studied the clinical and radiological outcome of 102 ABG1 stems inserted between September 1990 and December 1996.
Material: This prospective study included 91 consecutive patients (102 hips): 75 men and 27 women, mean age at surgery 63.1 years (range 34–81). The posterior approach was used for treatment of degenerative joint disease (82%) or osteonecrosis (11%). A size 5 or 6 stem was inserted in 60% of the cases. A metal-backed cup with a polyethylene insert was used in 45%, a zircona cup in 55%. The head diameter was 28 mm in 91% of the cases. At review, five patients had died, ten were lost to follow-up, eight responded to a phone interview, and x-rays were obtained for 79. Mean follow-up was 90.6 months (median 86.4 months).
Methods: The Postel Merle d’Aubigné (PMA) and Harris (HHS) scores were recorded for all patients. Position and quality of bone fixation were analysed. Stem migration was calculated on the different x-rays by calculating the distance between the centre of the head and the summit of the greater trochanter. Lucent lines (Gruen), osteolysis and the ARA score were noted. The SOFCOT 1997 radio-clinical classification was used. Actuarial survival curves were plotted taking stem revision as the endpoint. Quantitative data were compared with the chi-square test.
Results: The PMA pain score improved from 2.9 to 5.7 and the overall PMA score from 11.2 to 15.7. The HHS at last follow-up was 83.6 points. The stem was perfectly aligned in 81 cases, showed varus deviation in 26 and valgus deviation in four. A change from the initial alignment was noted in three cases and stem impaction in two. Six stem revisions were required for malposition (n=1), fracture during operation (n=3), or pain (n=2). Finally, there were no radiographic anomalies for 53 hips (class A), radiographic anomalies without clinical expression for 17 (class B), pain without detectable radiographic anomaly for six (class C), and poor clinical results with radiographic anomalies for four (class D). The ARA score was 5 or 6 in 51 cases. The 15-year actuarial survival rate was 88.9±4%.
Discussion: The form of the ABG1 implant allows an excellent alignment in only 80% of the cases. The metaphyseal anchor is achieved by bone remodelling of the calcar (23 atrophies including 10 severe cases)—to be differentiated from osteolysis (8 cases)—and an excessively transparent femur in the proximal 1A, B and 7 (12 cases). Transmission of force to the diaphyseal zone remains abnormal (30 thickenings with four overall thickenings) and two severe cases of diaphyseal necrosis.
Conclusion: The changes in the design of the ABG2 which has an under-dimensioned stem should improve shaft remodelling. The form of the implant and the favourable effect of hydroxyapatite coating responsible for bone remodelling in zone 1 and 7 (Gruen) should be given particular attention in the upcoming years.
