- Patient Position:
- ensure that patient has not rotated anteriorly, for this may allow acetabular component to be placed in retroverted position;
- ensure that patient's torso is not tilted inferiorly (as can occur from a bean bag), as this can cause reaming in an excessive verticle position;
- Acetabular Exposure
- an unobstructed view of the acetabulum is manditory;
- ensure that femur is retracted anteriorly to allow passage of reamers;
- if femur is inadequately retracted anteriorly, then it may force reamers posteriorly, and excessive reaming of posterior column will occur;
- carefully dissect the transverse acetabular ligament from its bony attachments anteriorly & post
- keep the blade superfical, to avoid brances of
obturator artery, which pass beneath it;
- Identification of Acetabular Floor:
- Reaming Direction: (see
adult femoral and acetabular anteversion and
component position);
- superior wall of the acetabulum is a slope that must be converted to a hemisphere;
- reaming is directed more against the medial and posterior wall than towards the acetabular roof;
- need to avoid plowing into the superior rim as larger reamers are used;
- goal is to avoid translating the center of rotation laterally or superiorly;
- usual goal is 20-30 deg of anteversion and 35-45 deg of abduction;
- w/ pt in lateral recumbent position after incision is made, one finger is placed in sciatic notch & one finger on anterior superior spine;
- w/ methylene blue, line is drawn on drapes between these 2 points;
- one line was drawn between sciatic notch & anterior spine on the drapes and a 2nd line was drawn and flexed an additional 10 degrees;
- w/ osteoarthritis: reaming is directed more centrally rather than peripherally;
- w/
protrusio:
- in severe acetabular protrusio reaming procedure is more aggressive and is directed peripherally and not centrally;
- therefore larger reamers and a larger cup is used;
- Reaming Depth:
- remember that the goal is to obtain near complete coverage of the acetabular component;
- use true floor of acetabulum as marker for depth of reaming;
- reaming to depth that obliterates this U shaped portion of acetabulum usually converts the bony acetabulum to hemisphere;
- initial reaming is done w/ instrument smaller than templated size;
- initial use of a large reamer can destroy the superior rim, necessitating a bone graft for adequate coverage;
- to avoid superior placement of acetabular component, initial reaming is directed more medially;
- w/ use of progressively larger reamers, one may find that transverse acetabular ligament is hypertrophic and must be excised to allow
larger reamers to enter the acetabulum;
-
subchondral bone:
- avoid reaming through the subchondral bone since this provides significant structural support;
- dense sclerotic bone in one region of acetabulum may result in eccentric reamining and eccentric cup placement;
- a hall burr is useful to remove small portions of sclerotic bone which is causing a malposition of the reamer;
- try to preserve subchondral bone, esp in superior aspect & periphery of acetabulum, but deepening acetabulum to obtain full seating of cup
in bone takes precedence over preserving subchondral bone;
- take care not to penetrate the medial wall, unless controlled penetration is necessary to obtain sufficient cup coverage;
- central portion of acetabulum requires more reaming than periphery;
- excess bone in inferior margin of acetabulum may later cause head of femoral component to become levered out of cup superiorly during adduction;
- if more bone must be removed from inferior margin of acetabulum, note occurance of significant bleeding from
obturator artery;
- references:
-
The value of preoperative planning for total hip arthroplasty. S. Eggli et al. JBJS. Vol 80-B. No 3. May 1998. p 382.
-
Medial Protrusio Tech for Placement of a Porous, Hemispherical Acetabular Component in a THR in Patients Who Have Acetabular Dysplasia
- Completion of Reaming:
- reaming is complete when all cartilage has been removed, reamers have cut bone out to periphery of acetabulum, and hemispheric shape has been produced;
- need to avoid damage to the posterior column w/ progressively larger reamers;
- note that w/ excessive anterversion the reamer will be directed into the posterior column and will "skive off" the anterior column;
- the surgeons hand nearest to the reamer, needs to keep a slight anterior directed force on the reamer so that it equally reams
the surfaces of the anterior and posterior acetabulum;
-
final reamer:
- final reamer is inclined firmly under the superior aspect of acetabular rim to ream the bone to a hemisphere from its normal sloped anatomy;
- careful not to remove bone from the superior rim at this step;
- final reamer size is determined by complete contact between the reamer and the acetabular rim
- use the last reamers in reverse inorder to expand the acetabulum (and compacting the underlying bone) w/o removing bone stock;
-
trial shells:
- trial shell that was the same size as the final reamer was then inserted;
- note final position of the shell in relation to the anterior and posterior walls (ie
acetabular component position)
-
consideration for oversized components:
- trial prosthesis is placed & evaluated for fit;
- if this trial shell can be buried down to its base with hand pressure, then next size of trial shell is inserted;
- optimal trial size will not "bottom out" with gentle tapping of the mallet;
-
final preparation:
- after satisfactory trial placement, acetabulum is further prepared by removing any remaining soft tissue & clearing out bone cysts;
- fill cysts with reamed bone graft and apply reamer in reverse to help impact the graft;
- Component Insertion:
- Case Example:
- 35 year old male w/ near anklyosed hip following a GSW to the hip;
- preoperative films appeared to indicate that little or no medialization was necessary;
- postoperative films, however, indicate that the cup was lateralized (hence, reaming was inadequate);
Position, orientation and component interaction in dislocation of the total hip prosthesis.
Inaccuracy of acetabular reaming under surgical conditions.
Reamed surface topography and component seating in press-fit cementless acetabular fixation.
Cementless Hemispheric Porous-Coated Sockets Implanted with Press-Fit Technique without Screws: Average Ten-Year Follow-up.
