There are some reports of patients who had displaced femoral neck fracture and were treated with BHA via the modified Dall approach [11]; however, to our knowledge, there is no report on greater trochanteric complications and size of the greater trochanteric fragment. A hypothesis was that the fragment was larger, so as type F was increased, and the fragment was smaller, so as type C was increased. However, there was no statistically significant difference for complications with the size of the greater trochanteric fragment. Knot position was greatly influenced of complications of the greater trochanter, if the knot position is standardized, the size of the fragment may be influenced the complications.
Oe et al. [12] reported that the incidence of greater trochanteric complications was 7.9% (37/466 hips) in their THA cases via the modified Dall approach using two UHMWPE fiber cables (applied when the greater trochanteric fragment was reattached). Kuroda et al. [13] also reported that the incidence of greater trochanteric complications was 5.0% (3/60 hips) in their THA cases via the modified Mostardi approach using two UHMPE fiber cables. In the present study, there were only a few greater trochanteric shifting cases (type C; 6.3%; 6/95 hips). The incidence of greater trochanteric fractures (type F) was 11.6% (11/95 hips) and almost identical to previous reports (Table 3). When the anterior and posterior knot outcomes were compared with previous findings, the outcome of the anterior knot was worse than past reports [12, 13]; however, the outcome of the posterior knot was identical to that reported by Oe et al. [12], although the patients were older and had femoral neck fracture. As the present study included patients with displaced femoral neck fracture and they were older, bone weakness was expected; thus, the incidence of greater trochanteric complications (greater trochanteric fracture in particular) were expected to be high. However, the result was equal to that of THA cases; thus, there was a confounding factor other than bone density.
Statistically, the age at operation, sex, size of the greater trochanteric fragment, type of the stem, and experience of the surgeon did not influence the greater trochanteric complications in the present study. In their THA cases performed via the modified Dall approach, Oe et al. [12] reported that the patients’ age and experience of the surgeon were risk factors for the complications of the greater trochanter. In the present study, the BMI and knot position were the risk factors (Table 2).
The incidence of greater trochanteric complications was significantly lower in the posterior group than in the anterior group. To our knowledge, no other institutions reported the difference in the knot position of the greater trochanteric fragment.
Although we could not examine such herein, the activity of daily living and strength of the gluteus medius muscle before the injury may influence the trochanteric complications; further examinations are necessary for THA cases.
The gluteus medius muscle can pull the fragment of the greater trochanter forward horizontally to the osteotomy surface; however, it is thought that the fragment digs into the ramp on the surface, which prevents the fragment from shifting anteriorly. When the knot was anteriorly positioned, the contact pressure with the tensor fascia lata muscle increased during external rotation, and it was thought that patients felt pain or discomfort and tended to assume the internal rotation position to avoid stimulation(Fig. 5). When the thighs assume the internal rotation position, the gluteus medius and gluteus minimus muscles contract and therefore pull the fragment horizontally to the osteotomy surface, causing the fragment to shift anteriorly. In addition, as the external rotation muscles become hyper-tense, the power to pull backwards increases, resulting in bone fracture at the non-cut area of the greater trochanter. Tip fracture occurs because the piriformis muscle and external rotator muscles, which lack plasticity, pull the tip of the greater trochanter strongly when the thighs assume the internal rotation position.
The limitations of the present study was that it was not a randomized study, it included relatively few cases, the follow-up period was short, it lacked data on clinical scores and functional outcomes, and the SC stem and Exeter stem were inserted depending on the surgeon’s discretion.