Introduction

Sagittal pelvic tilt (SPT) can change with spinal pathologies and fusion. Change in the SPT can result in impingement and hip instability. Our aim was to determine the magnitude of the SPT change for hip instability to test the hypothesis that the magnitude of SPT change for hip instability is less than 10° and it is not similar for different hip motions.

The position of this surgeon is that there is no approach that provides superior outcomes for total hip replacement (THR). The direct anterior approach (DAA) has become popular with patients because of marketing by companies, misinformation given to journalists for public consumption, and yes, some surgeons. Because of patient pressure generated by this marketing there has been pressure on surgeons to convert their surgical approach for perceived protection of their practice. Unfortunately, the leaders of orthopaedic organizations have not countered this marketing with education of the public that there is NO scientific evidence to support DAA superiority. These orthopaedic organizations exist to be advocates for their members but have abdicated that responsibility. Whatever happened to the time honored belief of choosing a surgeon to do your operation? Instead we now choose an approach? Do anterior surgeons think that they are immune to the Bell Curve of talent? The fact is that there is NO outcome data of DAA with the longest follow up study being one year, and recent data from both coasts of the USA raise concerns with more failures from loosening of the femoral component. How in the world can we bamboozle patients about better results when there are no published results with the DAA except for recovery? The mini-posterior approach has data for all aspects of its use. Short term data shows rapid recovery and hospital discharge can be the same day; gait studies show A quality at six weeks (so does this mean that cut muscles recover quickly?). Dislocation rates are equal in most comparative studies, but I believe this favors the DAA, however, fractures are 3X greater with DAA. Data from the Mayo Clinic comparative studies showed posterior patients return to work faster! There are two 10 year studies of mini-posterior patients which show some of the best 10 year results in the literature. And there are superior technical surgeons who perform this operation to the benefit of their patients, and they should not need to suffer the implicit bias from DAA marketing that their care of patients is inferior.

Dorr bone type is both a qualitative and quantitative classification. Qualitatively on x-rays the cortical thickness determines the ABC type. The cortical thickness is best judged on a lateral x-ray and the focus is on the posterior cortex. In Type A bone it is a thick convex structure (posterior fin of bone) that can force the tip of the tapered implant anteriorly – which then displaces the femoral head posteriorly into relative retroversion. Fractures in DAA hips have had increased fractures in Type A bone because of the metaphyseal-diaphyseal mismatch (metaphysis is bigger than diaphysis in relation to stem size). Quantitatively, Type B bone has osteoclastic erosion of the posterior fin which proceeds from proximal to distal and is characterised by flattening of the fin, and erosive cysts in it from osteoclasts. A tapered stem works well in this bone type, and the bone cells respond positively. Type C bone has loss of the entire posterior fin (stove pipe bone), and the osteoblast function at a low level with dominance of osteoclasts. Type C is also progressive and is worse when both the lateral and AP views show a stove pipe shape. If just the lateral x-ray has thin cortices, and the AP has a tapered thickness of the cortex a non-cemented stem will work, but there is a higher risk for fracture because of weak bone. At surgery Type C bone has “mushy” cancellous bone compared to the hard structure of type A. Tapered stems have high risk for loosening because the diaphysis is bigger than the metaphysis (opposite of Type A). Fully coated rod type stems fix well, but have a high incidence of stress shielding. Cemented fixation is done by surgeons for Type C bone to avoid fracture, and insure a comfortable hip. The large size stem often required to fit Type C bone causes an adverse-stem-bone ratio which can cause chronic thigh pain. I cement patients over age 70 with Type C bone which is most common in women over that age.

Short stems are an option for primary THR, but these are the technical challenges. Stem anteversion is increased with short stems usually above 20 degrees so cup anteversion must be adjusted lower. Offset is better if increased up to 5 mm more because more bony neck is retained and with increased stem anteversion the greater trochanter is more posterior, and both of these increase the risk of bony impingement. Short stems are best in A bone, okay in B bone, not recommended yet in C bone. With standard stems performing so well use caution for conversion to short stems.

Dislocation and accelerated wear have been the nemesis of hip surgeons. No study has been able to correlate cup position to instability. In recent years the influence of the spine-pelvis-hip construct has emerged as important to understand the shift in component position with postural change.

Using measurements familiar to spine surgeons, we have correlated the pelvic incidence (PI), a static measurement of pelvic width and hip position; the static tilt, a dynamic measure of pelvic-spine mobility. For THR we have measured the sagittal cup position as the fixed angular change of the cup shifts with pelvic tilt, and this is named anteinclination; and the sacral acetabular angle (SAA) which is the relationship of the acetabulum to the absolute value of sacral tilt (ST) in both standing and sitting. The pelvic femoral angle (PFA) is a measure of femur/hip flexion/internal rotation correlated to pelvic mobility.

Dislocation is most common in patients with low PI combined with an ST change <15 degrees. With normal PI and high PI, it occurs much less commonly and only in patients with ST change <5 degrees (very stiff). In patients with stiff pelvis (ST<13) the cup needs increased inclination and anteversion (45/20–25) to compensate for absence of cup opening by posterior tilt of pelvis. For patients with low PI and stiff pelvis we recommend constraint (such as dual mobility articulation).

A well designed constrained liner does not have a “hood” nor a wide poly brim that extends beyond the metal shell because these cause impingement. The failure of a good design is almost always technique.

Size the liner so the poly is press fit against the metal rim of the cup. Cement thickness does not matter. Remove any derotation tabs on metal rim with a carbide burr so there is a firm press fit with no toggle. Do NOT angle the poly to change the anteversion.

Use the carbide burr to scratch the inner surface of the cup and a soft tissue burr to scratch the backside of the poly.

Cement must be liquid enough to fully seat the poly against the metal rim. If cement too doughy it resists full seating.

Put metal ring in groove during implantation and cementing to prevent cement into the groove.

If this is a primary cup use screws with the cup or cement the poly into the acetabular bone.

Dry the head and inner surface of the poly to facilitated reduction. Align the head concentrically into the mouth of the poly and push simultaneously on the knee and over the greater trochanter.

Revision of M-O-M articulation:

Indications

Loose cup either radiographically or clinically. Clinical symptoms are persistent startup pain; straightening from the bent position; inability to do single limb stance; limp.

Unrelenting pain with any activity, even turning over in bed.

Soft tissue mass in groin or anterior hip (more common anterior to greater trochanter than posterior.

Elevated ion levels, especially cobalt. Elevated is 10µg/L but dangerous levels not defined (my definition is 40µg/L. Danger is cobalt poisoning. Also elevated ions almost always mean increased wear so local osteolysis and bone destruction is a risk with increased follow up.

Cobalt poisoning: objective findings are cardiopulmonary with increasing shortness if breath; second most common is cognitive change. (Memory loss, psychomotor retardation). Subjective finding is psychological effect of a poison in their body.

A cemented stem is certainly a good technique to choose for patients 75 years of age or older. In Europe, cemented stems remain prevalent for all ages. In my experience a patient with a cemented stem is comfortable sooner and the leg is also stronger sooner. Cement technique is the most important factor of a cemented stem and with good technique these stems have shown 30 years of longevity in published follow up studies.

Technical points: 1.) Broach only. No reaming. 2.) Maintain hard cancellous bone in the metaphysis. Do not keep weak, loose bone. Brush loose bone away. 3.) Irrigate the femur until the irrigant is clear. Pack it with absorbent gauze (we use Kerlix). 4.) Place a plug and insert cement with a gun and manually pressurise the cement until you feel strong back pressure. 5.) The stem should be press-fit into the cement to force interdigitation of cement into the bone. This means the cement cannot be liquid when the stem is inserted. It must be doughy. 6.) The cement mantle should be 2–3mm circumferentially so pick the correct stem size to permit that. A centraliser will help centralise the tip of the stem in the cement column and prevent the stem being against the edge of the bone which breaks the cement column.

When a constrained liner is used in a non-cemented cup it is advisable to add screw fixation to the cup even if the cup has an excellent press-fit because there is more pull-out pressure on the liner with a constrained cup. It also is necessary for the cup to be in the correct anteversion/inclination. It is not advisable to use a constrained liner to “make up” for poor cup position. The patient can still dislocate and then that will require an open reduction.

Our most common use with constrained sockets is to cement a liner into a well fixed cemented cup. We also will cement the liner into two-stage infections to keep it stable between those operations. Failure with the cemented liner into a non-cemented cup only occurs with poor surgical technique. There is only one correct surgical technique and violation of this can cause disassociation of the liner from the cup or dislocation of the head from the liner. The correct technique is: 1.) Preferably there is no hood on the liner because that can increase impingement. 2.) The liner size must have a press-fit of the liner edge to the edge of the metal shell. This is absolutely critical. The liner size cannot sink into the shell or be proud of the shell. 3.) The liner cannot be tilted in the shell to change anteversion or inclination. 4.) The backside of the polyethylene liner must be roughened with a high speed bur preferably in a spider web design. 5.) The inside of the cup should be roughened with a carbide bit of a high speed drill. The screw holes should be cleared of fibrous tissue. 6.) The cement thickness is not a critical factor and 1–2mm is always sufficient. 7.) Maintain pressure on the liner with one size smaller pusher (28mm for 32 inner diameter liner) until the cement is hard.

The acetabular component is the most troublesome implant. There is more written about acetabular placement than any other anatomical site. The problems are: Maintenance of center of rotation (COR); Coverage of the cup with correct inclination and anteversion; Maintenance of inclination below 50 degrees; Anteversion must be mated to the femur (combined anteversion).

COR is critical for balance of the correct offset and leg length. The inferior-medial metal edge of the cup should lie over the TAL or just proximal to it. No cortical bone of teardrop is palpable.

Coverage: Inclination of the normal bony acetabulum has a mean of 55 degrees (range to 70 degrees) so the posterior-superior edge of the cup may be uncovered in many hips to keep inclination below 50 degrees. 45 degrees is critical for wear and anteinclination. Anteversion of the cup is not independent of the femur (Brown/Callaghan, Hip Society Award paper). Femur anteversion must be known to precisely position the cup. Cup coverage is important here too. The posterior and anterior edges cannot be proud (may need to ream more medial).

Cementless fixation has become dominant for THR throughout the world, but are all stem geometries equivalent in results? Registries are the best source for studying this question because they are absent of personal bias. Results at 5 years allow separation of implants so this time frame was used. Comparing the same articulations (ceramic or metal-on- polyethylene), cementless stems with proximal enhancement (elliptical shape), or a tapered stem with rectangular cross section, have performed better than stems with a slim (blade) AP geometry. Almost all cementless stems reported to registries today are broached only tapered in design.

Introduction

Recent gains in knowledge reveal that the ideal acetabular cup position is in a narrower range than previously appreciated and that position is likely different based on femoral component anteversion. For that reason more accurate acetabular cup positioning techniques will be important for contemporary THA. It is well known that malalignment of the acetabular component in THA may result in dislocation, reduced range of motion or accelerated wear. Up to 8% of THA patients have cups malaligned in version by more than ±10° outside of the Lewinnek safe zone. This type of malalignment may result in dislocation of the femoral head and instability of the joint within the first year, requiring reoperation. Reported incidences of reoperation are 1-9% depending on surgical skills and technique. In addition, cup malalignment is becoming increasingly important as adoption of hard on hard bearings increases as the success of large head hard on hard bearings seems to be more sensitive to cup positioning. This study reports the accuracy of a haptic robotic system to ream the acetabulum and impact an acetabular cup compared to manual instrumentation.