PROTRUSIO AFTER MEDIAL ACETABULAR WALL BREACH IN TOTAL HIP ARTHROPLASTY Christopher T. Martin, MD1, Anneliese D. Heiner, PhD1,3, Thomas E. Baer, BA1, Andrew J. Pugely, MD1, Nicolas O. Noiseux, MD1,2

ABSTRACT Background: Medial protrusio is a recognized complication of total hip arthroplasty, but it is not known if a medial wall breach during cup implantation increases the risk. We thus investigated the effect of up to a 2 cm defect in the medial acetabular wall in a cadaveric model. Separately, we investigated the ability of acetabular screws to rescue the construct. Methods: Nine human fresh-frozen hemipelves were reamed medially to create the defect, implanted with acetabular cups, and then loaded to failure. The nine contralateral hemipelves were reamed in a standard fashion and ser ved as controls. Separately, nine hemipelves with a medial defect were augmented with two acetabular screws each, then loaded to failure, with the contralateral side as a control. Load-to-failure, stiffness, and energy were recorded. Findings: The presence of a medial wall defect decreased the load-to-failure by a mean of 26% (5710 v. 4221 N, p=0.024). The addition of two acetabular screws did not rescue the construct (mean 27% decrease, 4082 v. 2985 N, p=0.024). The majority of specimens failed in a supra-physDepartment of Orthopaedic Surgery and Rehabilitation, University of Iowa Hospitals and Clinics, Iowa City, IA, USA 2 Department of Orthopaedic Surgery, VA Medical Center, Iowa City, IA, USA 3 Department of Biomedical Engineering, University of Iowa, Iowa City, IA, USA Corresponding Author: Christopher T. Martin, MD Department of Orthopaedic Surgery University of Iowa Hospitals and Clinics 200 Hawkins Drive, 01008 JPP Iowa City, IA 52242 Email: [email protected] Conflict of Interest Statement: One or more authors (CM, NN, AP, AH) received funding in support of this article. The cost of purchasing the cadaveric specimens was supported by the Orthopaedic Research and Education Foundation. Supplemental funding was provided through a general donation to biomechanical research at the University of Iowa, made by Dr. Dan Fitzpatrick. Wright Medical (Wright Medical Technology, Inc., Arlington, TN) donated the implants. Ethical Review Statement: This study received an IRB exemption and was HIPAA compliant. 1

iologic range of force. Bone density correlated with failure loads (R2 range of 0.54-0.78), and osteoporotic specimens were more likely to fail at a physiologic range, consistent with forces experienced during minor stumbles or falls. Interpretation: Osteoporotic patients with a medial wall defect after hip arthroplasty may be susceptible to fracture during activities of daily living. Protected weight bearing with an assistive device may be reasonable in order to minimize fall risk until cup ingrowth is achieved. INTRODUCTION Prosthetic acetabular protrusio is a medial migration of the acetabulum cup past Kohler’s line and into the pelvis, and is a known complication of total hip arthroplasty1. The mechanism usually involves a periprosthetic fracture and subsequent pelvic discontinuity, with the incidence reported to be as high as 0.9%2. The defect typically causes pain and dysfunction necessitating revision arthroplasty, and severe cases with disruption of the intra-pelvic vessels and the bladder have been reported3. Furthermore, disruption of the intra-pelvic structures by the acetabular component can lead to sepsis if left untreated4. The risk factors for this complication include patient factors such as poor medial bone stock, pre-existing protrusio, and rheumatoid arthritis, as well as operative factors including both under and over-reaming of the acetabulum1,5-8. Under-reaming the acetabulum increases the contact stress between the acetabular rim and the prosthesis, thus increasing the risk of iatrogenic fracture at the time of cup impaction6,8. In contrast, excessive medial reaming causes a breach in the medial acetabular wall, and presumably predisposes the patient to fracture during activities of daily living post-operatively3. However, to the best of our knowledge only two prior biomechanical studies have investigated any effect of a medial wall breach5,9, with one being in canine pelves instead of human pelves5, and the results were conflicting. Some authors have proposed that the majority of the stability of the acetabular component comes from contact with the acetabular rim, and thus small medial defects might not have clinical significance9. Overall, the effect of a medial wall breach remains controversial, and very little biomechanical data exists to help guide decision making. Volume 35   99

C. T. Martin, A. D. Heiner, T. E. Baer, A. J. Pugely, N. O. Noiseux MATERIALS AND METHODS The devices and treatments described in this article have been approved by the Food and Drug Administration, and our article is compliant with the IRB requirements at our institution.

Figure 1: Right sided prosthetic protrusio that developed two weeks after a primar y total hip arthroplasty.

There are multiple treatment modalities available for the treatment of medial acetabular wall defects encountered at the time of a revision total hip arthroplasty, including trabecular metal buttons, revision cages, bone graft, and washers10-15. However, these treatment modalities are usually reserved for massive acetabular bone loss in revision cases. Ideally, if a small medial defect occurred during a primary hip arthroplasty, the surgeon should have an effective treatment modality to prevent the need for revision surgery entirely. At our own institution, a patient was recently referred with protrusio acetabuli, secondary to fracture, that developed only two weeks after her primary total hip arthroplasty (Figure 1). The initial treating surgeon had breached the acetabular wall during reaming, causing a two centimeter defect. After reviewing her case, we hypothesized that the medial wall disruption was responsible for the rapid failure, and that the use of well-placed acetabular screws may have strengthened the construct and prevented the subsequent medial displacement. To the best of our knowledge, no study has investigated the use of two prophylactic acetabular screws for restoring the strength of the acetabular construct after a breach in the medial wall has occurred. The purpose of this study was to test the biomechanical effect of a medial wall breach in a human cadaveric pelvis model, and to investigate the use of acetabular screws as an intra-operative treatment for this complication. We hypothesized that the presence of a medial breach would significantly decrease the load-to-failure strength. Furthermore, we hypothesized that adding two points of acetabular screw fixation would restore the stability of the construct.

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Specimens: We obtained 20 cadaveric pelvis specimens (40 hemipelves from Anatomic Gifts Registry (Hannover, MD)) with information regarding donor age, gender, and cause of death. Patients with known metastatic disease to the pelvis, or known traumatic pelvis injury were excluded. We also excluded two specimens after testing. The first was a 47 year-old female with a bone mineral density (BMD) of 643 mg/cm3, which was 185% higher than the mean BMD for the overall cohort. No results could be obtained because this specimen’s load at failure exceeded the maximum force that could be applied by our materials testing machine (15,000 N). The second excluded specimen was incorrectly potted, causing mechanical testing to prematurely abort. Overall, this left 18 specimens (36 hemipelves) that formed our study cohort. The average age of the included specimens was 75 years and the mean BMD was 327 mg/ cm3 (Table 1). The specimens were dissected free of all soft tissue attachments and wrapped in saline-soaked gauze to keep them moist when not being tested. All specimens were stored in plastic bags at -5 degrees Celsius, and were allowed to thaw overnight to room temperature prior to use. After being separated from the contralateral half, each hemi-pelvis was inspected with a lateral radiograph in order to look for fractures, tumors, malformations, or pre-existing deformity. No specimen had an obvious radiographic deformity. In addition, each specimen was scanned with a peripheral quantitative computed tomography scanner, which allowed precise measurements of bone mineral density (BMD). Experimental Groups: Specimens were stratified according to BMD and assigned to one of two treatment groups (Table 2), with assignment done in a block fashion in order normalize BMD between both groups. After stratification, the mean BMD in treatment group A was 320 mg/cm3 and in treatment group B was 334 mg/cm3, with no significant difference between groups (p=0.67). In both experimental groups, the treatment side was over-reamed medially until up to a two cm defect was created in the medial acetabular wall. The exact dimensions of the defect were measured for each specimen prior to testing (Table 1). The mean area of the created defect was similar between the two groups (299 mm2 v. 331mm2, p=0.52). A goal defect of two cm in diameter was chosen because that size of defect has previously been reported

Protrusio after THA Table 1: Description of Pelvic Specimens. Specimen Name

46

Control

46

387

A

20x15

50

518

84

CHF

548.6

 

 

 

 

474.1

C   C   C   C   C  

65

Lung Cancer

R L R L R L R L R L

F   F   F   F   F   F  

C

R

F

4

L

 

 

417.9

A

Control

50

74

Lung Cancer

319.4

A

Control

54

 

 

347.2

A

20x23

54

79

MI

205.6

A

15x18

52

207.2

A

Control

52

 

R

F

Failure to Thrive

299.9

A

Control

50

 

 

258.3

A

20x15

50

79

Respiratory Failure

254.3 225.5

Control 20x18

no

 

A A

 50

 

 50

77

Congestive Heart

230.4

A

15x19

50

 

 

Failure

230

A

Control

50

C

77

Sepsis

274.2

A

Control

46

275

A

22x18

46

406.4

B

15x15

54

450.2

B

Control

54

Liver Failure

414.2

B

Control

50

 

 

334.5

B

20x20

46

no no no no no yes no no yes no yes No yes

B

69

Renal Failure

323.9

yes

 

307.9

15x17 Control

46

 

B B

46

67

Lung Cancer

296.2

B

22x20

54

 

 

291.1

B

Control

54

71

Pulmonary

291.4

B

Control

54

no yes no no yes no yes no yes

74

Alzheimer’s

L

11

R L

F  

C

 

 

 

391.5

B

16x19

50

12

R

F

C

78

ESRD

394.8

B

Control

50

L R L R L R

  F   F   F

  C   C   I

 

 

329.3

B

15x20

50

67

Lung Cancer

356.6

B

Control

46

L

 

 

A 16

R

17

R L R L

F   F   F  

C   C   C  

13 14 15

18

L

Screws Implanted? no no no no no no no no no no no no

76

L

10

15x18

A

547.3

 

R

9

A

COPD

 

3

8

50

74

L

7

50

Control

C   C

F   M

6

18x18

A

Testing Group

R L R

5

A

BMD (mg/cm3)

Race

2

Cup Size (mm)

Cause of Death

Sex

1

Medial Wall Defect Size

Age (yr)

Side

62

 

Hypertension

248.9

B

18x14

54

100

CAD/MI

278.6

B

Control

50

 

 

233.5

B

20x20

50

77

ESRD

216.2

B

Control

50

 

 

201

B

20x20

50

Table 2: Description of Treatment Groups Treatment Groups

Defect Side

Control Side

Group A

Over Reamed, No Acetabular Screws

Normal Acetabular Cup Placement

Group B

Over Reamed, Two Acetabular Screws

Normal Acetabular Cup Placement

in patients with this complication16, and is consistent with the size of the defect seen in our own recent cases. On the contralateral, control side, the acetabulum was reamed line-to-line up to the medial acetabular wall with careful visual inspection and manual palpation to verify that no breach had occurred. Reaming was generally 1mm greater than the total diameter of the cup. The contralateral hemipelvis was chosen as the control for each experimental specimen in order to minimize Volume 35   101

C. T. Martin, A. D. Heiner, T. E. Baer, A. J. Pugely, N. O. Noiseux

A

B

C

Figure 2. (Left) A right sided specimen is shown prior to potting, with the positioning measured by the bubble level. (Right) A left sided specimen is shown in the MTS machine, with the ilium (A) and symphysis (B) both potted in PMMA.

variations in specimen age, bone mineral density, or bone morphology, and is a commonly used method of control5,17-19. In group A, no additional points of fixation were added on the over-reamed side. In group B, we used two acetabular screws to strengthen the construct on the over-reamed side. The screws were placed into the posterior-superior acetabular safe zone as defined by Wasielewski et al20. Screw depth was measured at the time of implantation and an appropriate length was chosen for each specimen. No additional points of fixation were added on the control side for either experimental group. The acetabular cups were inserted with 45 degrees of abduction and 15 degrees of anteversion, with local anatomic landmarks used to verify cup positioning. Wright Medical acetabular implants were used (Dynasty implant system, Wright Medical Technology, Inc., Arlington, TN). Cups ranging in size from 46mm to 60mm were available, and each specimen was custom fit with an appropriately sized implant (Table 1). Biomechanical Testing: For each hemipelvis, the table of the ilium and the pubic symphysis were separately potted with polymethylmethacrylate (PMMA), with the bones supplemented with multiple screws to increase PMMA purchase (Figure 2). To obtain uniform orientation of each specimen, a bubble level was screwed into the acetabular component (Figure 2 Left), to direct the potting of the ilium such that the ultimate force vector was positioned with the hip

102   The Iowa Orthopaedic Journal

in 15 degrees of abduction and 20-30 degrees of flexion, which correlates with the direction of the maximum load experienced during regular walking as defined by Bergman et al21. The pubic symphysis was then potted, with a stainless steel ball bearing on the undersurface to allow for multi-planar motion and rotation of the symphysis, separate from the ilium, during testing (Figure 2 Right). Each hemipelvis was attached to the load cell of an MTS Bionix 858 Materials Testing Machine (MTS Systems Corp., Eden Prairie, MN), through an x-y table. The acetabular cup was loaded through a femoral component attached to the actuator of the MTS (Figure 2 Right). We used a size 28mm femoral head with a Dynasty (Wright Medical Technology, Inc., Arlington, TN) femoral implant to deliver the load to the acetabulum, and the same femoral component was used in each test. Each specimen was first preconditioned with five cycles of axial loading at 0.5 Hz from 5 to 100 N, to ensure seating of all components. Each specimen was then loaded to failure at a rate of 0.2 mm/s; this loading rate was consistent with the rate chosen by other biomechanical studies of the acetabulum5,22-24. The tests were video recorded to assist with determination of failure modes. A load–displacement cur ve was generated for each specimen, and the maximum (failure) load was recorded (Figure 3). The displacement measurement of the MTS actuator was used in our calculations of energy-to-failure and initial stiffness for each test.

Protrusio after THA Table 3: Experimental Results Control Failure Load (N)

Defect Failure Load (N)

Percent Control

Control Stiffness (N/mm)

Defect Stiffness (N/mm)

Percent Control

Control Energy (J)

Defect Energy (J)

Percent Control

Mean

5710

4221

74

1630

1220

75

13

10

77

Std Dev

3565

2477

907

524

7

6

GROUP A (No screws)  

P value

GROUP B

0.024

 

 

0.124

 

 

0.033

 

 

Control Failure Load (N)

Defect Failure Load (N)

Percent Control

Control Stiffness (N/mm)

Defect Stiffness (N/mm)

Percent Control

Control Energy (J)

Defect Energy (J)

Percent Control

4082

2985

73

1239

903

73

10

9

90

296

401

4

6

 

0.124

 

 

0.554

 

(Two screws on the

Mean

defect side)

Std Dev

1251

1351

P value

0.024

 

 

Figure 4: Relationship between bone density and ultimate failure load.

Statistical Analysis: Specimens were compared against their control side with a paired-sample Wilcoxon signed rank test. Statistical significance was considered to be p

Protrusio After Medial Acetabular Wall Breach in Total Hip Arthroplasty.

Medial protrusio is a recognized complication of total hip arthroplasty, but it is not known if a medial wall breach during cup implantation increases...
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