Radiological analyses were performed on plain full weight bearing radiographs taken before surgery and at the three month follow-up visit or at the time at which any studied complication first occurred. Antero-posterior (AP) and medio-lateral (M/L) radiographs were used for analysis.
Various radiographic measurements have been used in assessing hip dysplasia at skeletal maturity (Laborie et al. 2010, Schofer et al. 2010). For this study, we selected some of the most commonly used and repeatable measurements to assess the preoperative radiographs (Figures 10-13). The radiographic teardrop in the antero-posterior view was a landmark for many measurements used in this study (Schofer et al. 2010). The inter-teardrop line was used as the transverse axis of the pelvis.
Figure 10. CE=Centre-edge angle is formed by vertical line through the centre of the femoral head and perpendicular to the transverse axis of the pelvis. Sharp's angle=describes the angle formed
between the inter-teardrop line and the line connecting the inferior tip of the teardrop to the acetabular rim.
Figure 11. The acetabular depth-width ratio (ADR) is the depth of the acetabulum (A) divided by the width of the acetabulum (B). The femoral head index (FHEI) quantifies how much of the femoral head is covered by the acetabulum (C/D x 100).
Figure 12. Horizontal (B) distance of the hip joint centre is measured from intersection of the distal end of the inter-teardrop line and from the line between the inferior edge of the sacroiliac joint and teardrop. Vertical (A) distance of the hip joint centre is measured between the centre of the femoral head and perpendicular to the inter-teardrop line.
Figure 13. Vertical (A) distance of the hip joint centre is measured between the centre of the femoral head and perpendicular to the inter-teardrop line. Horizontal (B) distance of the hip joint centre is measured from intersection of the distal end of the inter-teardrop line and from the line between the inferior edge of the sacroiliac joint and teardrop. Leg length discrepancy (LLD)
measurement (C) descripted by Kjellberg (Kjellberg et al. 2012). D is the rotation centre of the hip.
A standard hip offset measurement (E) was described by Woolson (Woolson et al. 1999). The centre-edge angle (F) is formed by a vertical line through the centre of the femoral head and perpendicular to the transverse axis of the pelvis.
Crowe classification was used to define DDH (Crowe et al. 1979). Crowe type I has <50%
subluxation, type II has between 50% and 74% subluxation, type III has between 75% and 99%
subluxation and type IV has a complete dislocation (Crowe et al. 1979).
Restoration of the leg length and femoral offset are important factors for the success of THA (Asayama et al. 2002, Clark et al. 2006). In order to perform the measurements as precisely as possible, pelvic landmarks and a detailed understanding of the measurements were ensured from prior analyses. The radiographic teardrop in the AP view was a landmark for many of the measurements used in our study (Vare 1952, Schofer et al. 2010, Laborie et al. 2011,). The inter-teardrop line was used as the transverse axis of the pelvis (Figures 10 and 12-13). LLD was evaluated from the preoperative and postoperative radiographs, where LLD was a perpendicular distance between a line passing through the lower edge of the inter-teardrop line to a line passing through the tip of the lesser trochanter (Figure 13) (Meermans et al. 2010, Kjellberg et al. 2011). Hip offset was measured with a standard method where hip offset is measured as the perpendicular distance between the centre of rotation of the femoral head and the femoral midline (Figure 13) (Woolson et al. 1999).
The centre-edge (CE) angle of Wiberg was measured (Wiberg et al. 1939). CE angle <20 degrees has been postulated to show hip dysplasia and >25 degrees to indicate a normal hip (Wiberg et al. 1939, Cooperman 2013). The acetabular roof angle of Tönnis was measured and the angle of inclination of the acetabulum, also known as Sharp's angle, was also measured (Sharp et al. 1961, Tönnis 1976). The normal range of the Sharp's angle is 33-38 degrees (Sharp et al. 1961). The acetabular depth-width ratio (ADR) was measured along a line running perpendicular from the width line to the deepest point of the medial arch (Cooperman et al.
1983). For the ADR, mean cut-off values are 0.235 for males and 0.233 for females (Cooperman et al. 1983). The femoral head extrusion index (FHEI) was measured to assess the degree of femoral head lateralisation over the acetabular edge (Heyman et al. 1950, Mast et al. 2010). The normal range of the FHEI was originally 70–100% but a cut-off value of 75% was subsequently proposed (Heyman et al. 1950, Cooperman et al. 1983). The anatomic hip centre was located from the pre- and postoperative radiographs by a method described by Fessy et al. (1999), which has been shown to be the most precise method for determination of the anatomic hip centre (Schofer et al. 2010) (Figure 12).
Proximal femoral measurements were made, and the Canal Flare Index (CFI) was calculated (Figure 14). The CFI expresses the shape of the proximal femur; CFIs <3.0 are described as stovepipe-shaped canals, 3.0–4.7 as normal canals, and 4.8–6.5 as champagne flute-shaped canals (Noble et al. 1988). Femurs were qualitatively assessed based on three distinct patterns of the shape and bone structure of the proximal femur (Dorr et al. 1993). Type A femurs had thick medial and lateral cortices on anterior-posterior radiographs and a large posterior cortex on lateral radiographs. Thick diaphyseal cortices render the proximal femur funnel-shaped.
Type B femurs exhibited bone loss from the medial cortex on anterior-posterior radiographs and from the posterior cortex on lateral radiographs. The intramedullary canal of Type B femurs was wider than that in Type A femurs. Type C femurs lost nearly all medial and posterior cortices; they were thin and may have displayed a fuzzy appearance on radiographs.
The intramedullary canal diameter was usually wide on lateral radiography. Anterior-posterior cortical index, mediolateral cortical index, and the canal-to-calcar ratio were measured (Figure 14) (Dorr et al. 1993).
Figure 14. Radiological measurements of the proximal femur according to Noble: A, femoral head offset; B, femoral head diameter; C, femoral head position; D, canal width 20 mm above the mid-lesser trochanter line; E, canal width at the mid-mid-lesser trochanter line; F, canal width 20 mm below the mid-lesser trochanter line; G, isthmus diameter; H, isthmus position below the mid-lesser trochanter line; I, neck-shaft angle. B. Radiological canal flare index (CFI) measurements of the proximal femur according to Noble: A, canal width +20 mm above the mid-lesser trochanter line;
B, isthmus diameter. CFI = A / B. C. Radiological measurements of the proximal femur: canal-calcar ratio (X / Y) and cortical index ((Z − X) / Z).
Migration of the femoral component during follow-up was evaluated by measuring the distance from the tip of the greater trochanter to the femoral component shoulder based on post-operative radiography and from the last obtained radiograph after calibration.
The functional structure of the acetabulum changes due to osteoarthritis. It has been stated that the fixation of the cementless acetabular component is related to the bone structure of the acetabulum (Dorr et al. 2000). We classified the acetabulum into three types (Type A, Type B and Type C) based on the roof morphology on plain preoperative radiographs according to Dorr et al. (2000). The Type A acetabulum has an isosceles triangle with equal medial and lateral walls or beams and a shorter base. Type B has an extend triangle which has a pseudopod that extends into the teardrop and creates a thick medial wall. Type C is found only in dysplastic hips and does have a right-angle triangle with a straight lateral wall and the femoral head may or may not be located under the triangle (Dorr et al. 2000).
Acetabular component inclination and anteversion angles were measured from the postoperative radiographs. The inclination angle of the acetabulum component was measured according to the method described by Widmer (2004). Anteversion was measured according to the method described by Murray (1993). The number of acetabular components which were positioned within the safe zone (5–25° anteversion and 30–50° inclination) were analysed, as described by Lewinnek et al. (1978).
The acetabulum components were divided into four groups based on the amount of containment of the cup described by Sarmiento et al. (1990). Containment was measured from the postoperative radiographs. Containment by bone was recorded as i) 100%, ii) 90–99%, iii) 75–99% and iv) <75%. Radiolucent gaps on the initial postoperative radiograph and radiolucent lines or osteolysis at the bone component interface on the subsequent radiographs were recorded as described by DeLee et al. (1976).
Picture archiving and communication systems (PACS) were used in every participating hospital. Agfa IMPAX (ver. 6.5.2.657) PACS was used at Helsinki University Hospital and Sectra Workstation IDS7 (ver.15.1.8.5) PACS were used at Kuopio University Hospital and Turku University Hospital.