High-yield overview

Acetabular component addition ± stem revision | Advanced

Primary Indications for Conversion

Indication	Presentation	Urgency
Acetabular erosion	Groin pain, medial migration on XR	Semi-elective
Recurrent dislocation	Multiple dislocations, instability	Semi-elective
Stem loosening	Thigh pain, stem subsidence	Elective
Periprosthetic fracture	Acute pain, unable to weight-bear	Urgent
Infection	Pain, systemic symptoms, elevated CRP	Staged/urgent

Timing of Conversion

Average time to conversion is commonly reported at around 6-8 years; cohort series (e.g. Diwanji, mean 7.2 years) report similar intervals
A substantial proportion of conversions occur within the first 5 years
Late presentation may have severe erosion/protrusio

Patient Factors Favouring Primary THA

Age less than 70 years (cognitive function)
High functional demand
Pre-existing hip arthritis
Long life expectancy

Mnemonic

GROIN

Causes of Pain After Hemiarthroplasty

Mnemonic

TAPER

Stem Retention Checklist

Critical Danger Structures

Sciatic Nerve

Lies 2-3cm posterior to hip capsule, at increased risk in revision surgery due to scarring - May be adherent to capsule/scar tissue. Traction injury during leg lengthening (greater than 4cm). EXAM KEY: Identify nerve early if posterior approach, limit lengthening to 4cm, flex knee during retraction, consider nerve monitoring.

Femoral Neurovascular Bundle

Lies anterior to hip capsule, 4-5cm from acetabulum - At risk with aggressive anterior retraction, especially if medialized acetabulum with erosion. EXAM KEY: Anterior retractors on bone only (not soft tissue), be aware of protrusio bringing medial wall closer to vessels.

Superior Gluteal Nerve

Exits greater sciatic notch and runs with superior gluteal vessels 3-5cm above acetabular rim - Injury causes abductor weakness and Trendelenburg gait. EXAM KEY: Do not extend split in gluteus medius more than 5cm from tip of greater trochanter.

Medial Acetabular Wall

Often eroded/paper-thin in conversion cases, may have protrusio - Aggressive reaming risks medial wall perforation and injury to intrapelvic vessels (obturator, iliac). EXAM KEY: Assess medial wall on CT preoperatively, ream carefully, consider medialised cup design.

Femoral Shaft

Cement mantle and osteoporotic bone increase fracture risk during stem removal - Cemented stems require careful extraction. EXAM KEY: If stem revision needed for cemented stem, consider extended trochanteric osteotomy. Have plating and cerclage available.

Pre-operative Planning

Acetabular Assessment

Centre of Rotation (CoR)
- Compare to contralateral normal hip
- Erosion causes medial and superior migration
- Plan to restore anatomic CoR if possible

Bone Stock Assessment (Paprosky)

Type	Description	Treatment
I	Supportive rim, minimal bone loss	Standard cementless cup
IIA	Superior dome loss, less than 30%	Cementless cup, screws
IIB	Less than 50% superolateral loss	Cementless cup, possible augment
IIC	Medial wall deficient	Medial augment or protrusio cup
IIIA	Greater than 40% host bone contact	Jumbo cup, augments
IIIB	Less than 40% host bone contact	Cage, cup-cage, custom triflange

Templating
- Template cup size and position
- Assess head size needed for stability
- Plan offset and leg length restoration

Femoral Assessment

Stem Fixation
- Subsidence on serial radiographs
- Radiolucent lines (complete vs partial)
- Endosteal sclerosis (stable fibrous fixation)
Stem Type Identification
- Manufacturer and model
- Cemented vs cementless
- Modular vs monoblock
- Collar presence
Taper Compatibility
- Confirm manufacturer has THA head for this stem
- Document taper dimensions (12/14, V40, etc.)
- If uncertain, plan for stem revision

Operative Technique: Stem Retention

Step 1: Positioning and Approach

Patient lateral decubitus. Use previous incision - typically posterior approach for hemiarthroplasty. May need to extend proximally or distally for exposure. Prepare for extensile approach if needed.

Clinical Pearl

Approach Selection: Use the previous approach. If lateral approach used for original hemiarthroplasty, can still use posterior for conversion if preferred - just be aware of previous scar and gluteal insertion.

Step 2: Exposure and Dislocation

Identify and protect sciatic nerve (if posterior approach). May be embedded in scar tissue - careful blunt dissection. Release short external rotators (tag for repair if posterior approach). Capsulectomy around femoral neck and acetabulum. Dislocate hip with internal rotation and flexion.

Sciatic Nerve Protection

Nerve may be adherent to scar tissue. Palpate nerve before any sharp dissection posterior to hip. Keep knee flexed during retraction to reduce tension.

Step 3: Femoral Component Assessment

With hip dislocated:

Remove hemiarthroplasty head (modular) - note taper condition
Test stem stability: rotational torque, axial stress
If ANY toggle or rotation, stem requires revision
Inspect taper for corrosion, fretting, or damage
If taper damaged, stem requires revision

Clinical Pearl

Taper Inspection: Clean taper with saline, dry thoroughly, use magnification or loupe. Any visible corrosion or pitting = revise stem. Corrosion products cause ALTR (adverse local tissue reaction).

Step 4: Acetabular Exposure

Elevate labrum and capsular remnants. Identify transverse acetabular ligament. Place retractors on anterior wall (protected) and posterior wall. Assess acetabular cartilage wear and bone stock.

Step 5: Acetabular Preparation

Remove remaining articular cartilage with curettes. Ream from peripheral rim - do NOT follow eroded medialized centre of rotation. Sequential reaming to healthy bleeding bone. Assess rim for defects.

Reaming Caution

Medial wall may be paper-thin from erosion. Do NOT breach medial wall. If protrusio present, may need bone graft or medialised cup design rather than aggressive reaming.

Step 6: Acetabular Component Insertion

Standard Bone Stock (Paprosky I-IIA)

Cementless press-fit cup
Line-to-line or 1-2mm under-ream
Screws for supplemental fixation
Target: 40-45° inclination, 15-20° anteversion

Medial Wall Deficiency (Paprosky IIC)

Consider protrusio ring or medialised cup design
Bone graft medial defect if large
May need larger cup to span defect

Significant Bone Loss (Paprosky III)

Trabecular metal augments
Jumbo cups
Cup-cage constructs
Consider reconstruction cage

Step 7: Head Selection and Trialling

Select THA head compatible with retained stem taper. Trial different neck lengths for:

Stability (no impingement, adequate tension)
Leg length (equal to contralateral)
Offset (adequate abductor tension)

Consider larger head (36mm) and/or dual mobility for instability risk.

Step 8: Final Reduction and Closure

Place final head with single firm impaction. Reduce hip. Test stability in flexion/IR (posterior) and extension/ER (anterior). Document ROM achieved. Repair posterior structures if posterior approach. Layered closure over drain.

Operative Technique: Stem Revision

When Stem Revision Required

Stem loosening (subsidence, toggle)
Monoblock head (non-modular)
No compatible THA head available
Taper corrosion or damage
Malposition (version, offset, leg length)

Step 1: Head Removal

For monoblock stems, apply axial traction and rotation to remove entire stem with head.

For modular stems with incompatible/corroded taper:

Remove modular head using head extractor
Inspect taper - if corroded, must revise stem

Step 2: Cemented Stem Removal

If cemented hemiarthroplasty:

Extended Trochanteric Osteotomy (ETO)
- Mark 10-12cm from tip of trochanter distally
- Oscillating saw for longitudinal cuts
- Thin osteotome to complete
- Protects from perforation, allows cement removal
Cement Removal
- Remove bulk cement with osteotomes
- Ultrasonic cement removal device
- High-speed burr for residual cement
- Remove all cement to distal extent
Canal Preparation
- Sequential broaching
- Revision stem (modular, fully porous)

Step 3: Cementless Stem Removal

Interface disruption
- Flexible osteotomes around stem
- ETO if extensively ingrown
Extraction
- Stem extractor with axial pull
- May need trephine for bony ingrowth

Step 4: Revision Stem Insertion

Select stem based on remaining bone:

Metaphyseal fixation if diaphysis intact
Diaphyseal fixation (fully porous) if metaphysis deficient
Modular stems allow independent offset/length adjustment

Secure ETO with cerclage wires if used.

Dual Mobility and Instability Prevention

Risk Factors for Dislocation

Factor	Risk Level	Management
Previous dislocation	High	Dual mobility or constrained
Abductor deficiency	High	Dual mobility
Cognitive impairment	Moderate	Consider constrained
Posterior approach	Moderate	Repair posterior structures
Revision surgery	Moderate	Consider dual mobility
Neuromuscular disease	High	Constrained liner

Dual Mobility Cup

Inner head articulates with mobile polyethylene liner
Outer liner articulates with metal shell
Increases effective head size and jump distance
Reduces dislocation to less than 1% in revision settings in registry and cohort series
Registry evidence (Swedish Hip Arthroplasty Register) supports low re-revision-for-dislocation rates when used for instability

Constrained Liner

Capture mechanism locks head into liner
Reserved for highest risk cases
Higher stress on fixation interface
Requires excellent cup fixation

Clinical Pearl

Dual Mobility Indication: Consider for ALL conversion THA cases given inherently higher dislocation risk. Especially indicated if: previous dislocation, abductor weakness, cognitive impairment, or cannot comply with precautions.

Conversion THA Complications

Clinical Decision Scenarios

Use these scenarios to practise clinical reasoning and management decisions

CLINICAL SCENARIOStandard

CLINICAL PROMPT

"A 75-year-old woman presents with progressive groin pain 7 years after a cemented hemiarthroplasty for a displaced intracapsular femoral neck fracture. Her inflammatory markers are normal. How do you assess and manage this patient?"

PRACTICAL APPROACH

This presentation is highly suggestive of acetabular cartilage erosion, the most common indication for conversion of hemiarthroplasty to THA. My structured assessment would proceed as follows: **History**: Confirm groin pain (typical for acetabular pathology) rather than thigh pain (stem loosening) or buttock pain (referred from spine). Assess functional impact - walking distance, need for aids, effect on ADLs. Document any history of dislocation or instability. Assess medical comorbidities and fitness for surgery. **Examination**: Assess gait for antalgic pattern or Trendelenburg. Check ROM - typically preserved until late. Look for leg length discrepancy. Assess abductor strength. **Investigations**: AP pelvis and lateral hip radiographs would show medial migration of the femoral head, narrowing of the 'joint space' (cartilage wear), and possibly protrusio. I would compare to contralateral hip and original post-operative films. ESR and CRP to exclude low-grade infection. CT scan to assess acetabular bone stock (Paprosky classification) and confirm femoral stem fixation. I would identify the stem manufacturer and confirm availability of compatible THA heads. **Management**: Assuming infection excluded and she is fit for surgery, I would recommend conversion THA. My key decisions: *Stem Decision*: If the stem is well-fixed (no subsidence, no radiolucent lines), modular (can remove head), and I can confirm taper compatibility with available THA heads, I would retain the stem. If there is ANY loosening, it's monoblock, or no compatible head available, I would revise the stem - in a cemented stem, this would require extended trochanteric osteotomy. *Acetabular Approach*: Based on CT bone stock assessment, plan appropriate reconstruction. Paprosky I-IIA would get standard cementless cup. IIC or worse may need augments or specialised constructs. *Stability Considerations*: Given the 5-10% dislocation rate in conversion THA, I would strongly consider a dual mobility cup, especially if there are additional risk factors (cognitive impairment, abductor weakness). I would counsel her that conversion THA has higher risks than primary THA but typically achieves good pain relief and functional improvement.

CLINICAL SCENARIOStandard

CLINICAL PROMPT

"During conversion THA, you retain the well-fixed stem and insert the acetabular component. At trial reduction, the hip feels unstable with the shortest head option. How do you troubleshoot this?"

PRACTICAL APPROACH

Hip instability at trial reduction in a conversion THA requires systematic troubleshooting. I would work through the following algorithm: **Step 1: Confirm Component Position** *Acetabular Component*: - Check inclination (target 40-45°) - excessive inclination reduces coverage - Check anteversion (target 15-20°) - combined anteversion with stem - Use intraoperative radiograph or fluoroscopy if uncertain - If malpositioned, remove and reposition cup *Femoral Component*: - Retained stem position is fixed - If stem is anteverted AND cup anteverted, may be excessive combined anteversion - If stem position unfavourable, may need to revise stem despite initial plan to retain **Step 2: Soft Tissue Considerations** - Assess abductor competency - may be deficient from original trauma/surgery - Check posterior structures if posterior approach - ensure these will be repaired - Assess for excessive capsular laxity **Step 3: Head/Neck Options** With shortest head still unstable: - Consider extended offset liner to increase head coverage - Consider larger head size (36mm) if liner accommodates - Consider elevated rim liner (position to cover instability direction) **Step 4: Dual Mobility Cup** If still unstable despite above measures, I would: - Remove the liner and trial head - Insert dual mobility liner compatible with the shell - Dual mobility dramatically increases effective head size and jump distance - This is an excellent salvage for conversion THA instability **Step 5: Consider Stem Revision** If instability is due to: - Excessive stem anteversion (combined anteversion greater than 50°) - Inadequate offset from the retained stem - Subsidence discovered during stability testing Then stem revision may be necessary despite original plan to retain. **My Approach in This Case**: Given this is conversion THA with inherent high dislocation risk, if there is ANY instability at trial, I would convert to dual mobility rather than proceed with marginal stability. Dual mobility reduces dislocation risk to less than 1% and is my preferred solution for this scenario.

CLINICAL SCENARIOStandard

CLINICAL PROMPT

"A 68-year-old man had a cemented Thompson hemiarthroplasty 5 years ago. He now presents with groin pain and thigh pain. Radiographs show acetabular erosion AND stem subsidence of 8mm compared to immediate post-operative films. How do you manage this case?"

PRACTICAL APPROACH

This patient requires conversion THA with STEM REVISION. The combination of acetabular erosion (groin pain) and stem loosening (thigh pain, 8mm subsidence) means both components need to be addressed. Additionally, Thompson hemiarthroplasty is a MONOBLOCK design - the head is not removable, mandating stem revision regardless of fixation status. **Pre-operative Planning**: *Acetabular*: CT scan to assess bone stock using Paprosky classification. The erosion and likely medialized CoR will require careful reconstruction. Template cup size and position. *Femoral*: Thompson stems are cemented monoblock. I will need: - Extended trochanteric osteotomy (ETO) instruments - Cement removal equipment (ultrasonic, high-speed burr) - Revision stem options (modular or fully porous coated) - Cerclage wires for ETO fixation **Operative Technique**: *Step 1: Exposure* Previous posterior incision. May need extensile exposure given complexity. Protect sciatic nerve carefully. *Step 2: Extended Trochanteric Osteotomy* ETO is essential for safe cemented stem removal: - Mark osteotomy 10-12cm from tip of trochanter - Longitudinal cuts with oscillating saw on anterolateral and posterolateral cortex - Complete with thin osteotomes - Hinge on soft tissue laterally - This provides direct access to cement mantle and protects from perforation *Step 3: Cement Removal* - Remove bulk cement with osteotomes - Ultrasonic cement removal for adherent cement - High-speed burr for residual - Remove all cement to distal extent - Curette to remove debris *Step 4: Canal Preparation* - Sequential broaching - Choose revision stem based on remaining bone stock - May need distal fixation if metaphysis deficient *Step 5: Revision Stem Insertion* - Press-fit stem to achieve rotational stability - Restore offset and leg length using modular neck if needed - Secure ETO with cerclage wires (typically 2-3) *Step 6: Acetabular Reconstruction* - Ream to healthy bleeding bone - Insert cementless cup with screws - Augments if bone deficiency - Consider dual mobility given revision complexity *Step 7: Final Reduction* - Trial for stability, leg length, offset - Final components - Repair posterior structures **Post-operative Management**: - Protected weight-bearing until ETO healed (typically 6-8 weeks) - Serial radiographs to monitor ETO and stem - Standard thromboprophylaxis (extended to 35 days) - Hip precautions 6-12 weeks

Evidence Base

Total hip arthroplasty after failed internal fixation of proximal femoral fractures

Level IV

Archibeck MJ, Carothers JT, Tripuraneni KR, White RE Jr • J Arthroplasty

Clinical Implication: Salvage THA after failed prior fracture surgery is clinically successful but carries an elevated risk of periprosthetic fracture and dislocation versus primary THA - counsel patients and plan for stability and femoral protection accordingly.

Verify on PubMed (PMID 22682040)

Clinical results of conversion total hip arthroplasty after failed bipolar hemiarthroplasty

Level IV

Diwanji SR, Kim SK, Seon JK, Park SJ, Yoon TR • J Arthroplasty

Clinical Implication: Conversion THA reliably relieves pain and restores function after failed hemiarthroplasty, but its perioperative complication profile approximates that of revision rather than primary THA.

Verify on PubMed (PMID 18534504)

The risk of revision due to dislocation after total hip arthroplasty depends on surgical approach, femoral head size, sex, and primary diagnosis (78,098 operations, Swedish Hip Arthroplasty Register)

Level III

Hailer NP, Weiss RJ, Stark A, Karrholm J • Acta Orthop

Clinical Implication: Fracture-related diagnoses and small head sizes are independent dislocation risk factors - supporting larger heads or dual-mobility constructs and careful approach selection in the conversion/fracture population.

Verify on PubMed (PMID 23039167)

Dual-mobility cups for revision due to instability are associated with a low rate of re-revisions due to dislocation: 228 patients from the Swedish Hip Arthroplasty Register

Level III

Hailer NP, Weiss RJ, Stark A, Karrholm J • Acta Orthop

Clinical Implication: Dual-mobility cups markedly reduce recurrent instability when used for revision/conversion THA at high dislocation risk, justifying their liberal consideration in this setting.

Verify on PubMed (PMID 23116439)

A dual-mobility cup reduces risk of dislocation in isolated acetabular revisions

Level IV

Civinini R, Carulli C, Matassi F, Nistri L, Innocenti M • Clin Orthop Relat Res

Clinical Implication: When the well-fixed stem is retained and only the acetabulum is reconstructed, a dual-mobility cup provides stability without compromising fixation - directly relevant to stem-retaining conversion THA.

Verify on PubMed (PMID 22700131)

References

National joint replacement registries (NJR England/Wales, AJRR USA, AOANJRR Australia, SHAR Sweden, NZJR New Zealand). Annual reports. Used as global registry evidence for revision and dislocation rates after conversion/revision THA.
Bhandari M, Devereaux PJ, Tornetta P 3rd, et al. Operative management of displaced femoral neck fractures in elderly patients. An international survey. J Bone Joint Surg Am. 2005;87(9):2122-2130.
Archibeck MJ, Carothers JT, Tripuraneni KR, White RE Jr. Total hip arthroplasty after failed internal fixation of proximal femoral fractures. J Arthroplasty. 2013;28(1):168-171. PMID 22682040.
Tidermark J, Ponzer S, Svensson O, et al. Internal fixation compared with total hip replacement for displaced femoral neck fractures in the elderly. A randomised, controlled trial. J Bone Joint Surg Br. 2003;85(3):380-388.
Diwanji SR, Kim SK, Seon JK, et al. Clinical results of conversion total hip arthroplasty after failed bipolar hemiarthroplasty. J Arthroplasty. 2008;23(7):1009-1015. PMID 18534504.
Parvizi J, Picinic E, Sharkey PF. Revision total hip arthroplasty for instability: surgical techniques and principles. J Bone Joint Surg Am. 2008;90(5):1134-1142.
Hailer NP, Weiss RJ, Stark A, Kärrholm J. The risk of revision due to dislocation after total hip arthroplasty depends on surgical approach, femoral head size, sex, and primary diagnosis. An analysis of 78,098 operations in the Swedish Hip Arthroplasty Register. Acta Orthop. 2012;83(5):442-448. PMID 23039167.
Civinini R, Carulli C, Matassi F, et al. A dual-mobility cup reduces risk of dislocation in isolated acetabular revisions. Clin Orthop Relat Res. 2012;470(12):3542-3548. PMID 22700131.
Hailer NP, Weiss RJ, Stark A, Kärrholm J. Dual-mobility cups for revision due to instability are associated with a low rate of re-revisions due to dislocation: 228 patients from the Swedish Hip Arthroplasty Register. Acta Orthop. 2012;83(6):566-571. PMID 23116439.
Younger TI, Bradford MS, Magnus RE, Paprosky WG. Extended proximal femoral osteotomy. A new technique for femoral revision arthroplasty. J Arthroplasty. 1995;10(3):329-338. PMID 7673912.