Several risk factors were identified for periprosthetic acetabular fractures. These include significant osteoporosis and other impairments of bone quality, the local individual anatomy with potential periacetabular bone defects, osteolysis, rheumatic arthritis, the type of surgical procedure, especially poor cup positioning, the chosen implant, and the age of patients (younger/active patients with higher risks), respectively.⁶

25.1 Epidemiology

Periprosthetic acetabular fractures are less common than femoral fractures. Their prognostic relevance is based on the integrity and stability of the posterior column.⁷

Periprosthetic acetabular fractures can occur intraoperatively, in the early postoperative period, after relevant trauma and due to stress osteolysis.⁶ Additionally, pathologic conditions involving the bone such as osteoporosis, severe osteolysis, infection, cancer, and Paget disease can reduce its structural integrity.⁸

Historically, an analysis from 1974 reported a rate of intraoperative fractures of less than 0.02% after cemented cups, corresponding to a risk of 1:5400.⁹ After cementless cup insertion, this risk is slightly increased to 1:1490 with a rate of 0.07%.¹⁰ Biomechanical data indicate a theoretically higher fracture risk after cementless press fit anchoring^{11,​12,​13} due to the high impaction forces during insertion.

A recent analysis showed higher intraoperative rates of 0.4% after uncemented cup insertion,¹⁴ whereas no acetabular fractures were observed using cemented cups. After elliptic monoblock cups, the rate was lowest at 0.09%; after hemispherical modular components, the rate was significantly higher with 3.5%.¹⁴

Data from the Mayo Clinic report either rate after uncemented and cemented cups, considering primary and revision total hip replacement (THR).¹⁵ The following rates were reported:

• 
  

  0.43% primary THR, uncemented cup

  
  
• 
  

  0.05% primary THR, cemented cup

  
  
• 
  

  0.74% revision THR, uncemented cup

  
  
• 
  

  0.53% revision THR, cemented cup

Overall, after primary THR, a rate of 0.24% was found, and after secondary/revision THR a rate of 0.68%—nearly threefold higher—was reported.¹⁵

Additionally, the type of the cup seems to influence the fracture rates. After insertion of uncemented elliptical cups, higher fracture rates were observed in comparison to hemispherical cups.^11,​16

In an experimental study by Kim et al, under-reaming of uncemented cups for press-fit insertion was identified as the main risk factor.¹⁶

In contrast, presently no clear data are available regarding postoperative fracture rates after primary and revision THR. Benazzo stated that 3.8% of hip revisions are due to acetabular fractures, with a higher rate expected in the near future.^8,​17

Recently, first data of 32,684 primary THR and 5435 revision THR were reported from the Mayo Clinic.¹⁵

The fracture rate after primary THR was 0.6%, with 0.5% after uncemented and 0.64% after cemented cups. Correspondingly, the rate after revision THR was significantly higher (1.5%, with 1.5% after uncemented and 1.3% after cemented cups).¹⁵

In patients younger than 30 years, this rate seems to be higher with 1.7%.¹⁸

A higher rate is suspected in specific bone conditions. In a systematic literature overview, fracture rates of 2–5% were reported in patients with rheumatoid arthritis.¹⁹

25.2 Injury Mechanism

In the acute phase, these fractures can occur intraoperatively while inserting a press-fit cup or while screwing a cup or during excessive acetabular reaming.

In the direct postoperative period, traumatic falls or accidents can result in acetabular fractures. Thus, traumatic fractures are often observed.

In the later course, slowly developing periacetabular fractures can be the result of cup loosening,²⁰ often after polyethylene wear-induced osteolysis, which limits the overall bone quality up to relevant bone defects. In all cases of osteolysis metal-on-metal reactions, corrosion or infection should be considered.

In addition, creeping acetabular fractures were reported after both cementless and cemented cup implantation, and were associated with preexisting risk factors such as osteoporosis.²¹

Lyons et al divided these fracture-related factors into technique-related factors and implant-related factors.²² The identified technique-related factors are revision procedures, bone defects, oversized cup components, and under-reaming, whereas implant-related factors are press-fit insertion and uncemented cups.

25.3 Diagnostics

Regarding diagnostics, intraoperative fractures have to be distinguished from posttraumatic and creeping fractures.

25.3.1 Intraoperative Fractures

Intraoperative detection of periacetabular fractures needs a high index of suspicion as these fractures are often overlooked. An experimental analysis showed that, of 18 fractures, only 15 fractures were detectable radiographically.¹⁶ Clinical experience confirmed these results with a 30% rate of overlooked fractures.¹³ Thus, intraoperative radiographs are mandatory.²³ A sudden change in resistance, suspected implant instability, an unusual noise, or changes in clinical examination can be signs of potential fracture and should be followed by at least intraoperative fluoroscopy. In all doubtful situations, an early postoperative CT is recommended.

Often, early postoperative plain X-rays show fracture signs. As oblique views are often insufficient to analyze the fracture lines, CT is always recommended, potentially with additional CT angiography.^{6,​10,​24}

In all other cases, a detailed history and clinical examination are the basis for further diagnostics and treatment recommendations.

25.3.2 Posttraumatic Fractures

Analysis of the suspected injury/fracture mechanisms can distinguish between adequate or inadequate trauma.²¹ In the latter, a missed intraoperative fracture should be considered.¹

Acute posttraumatic fractures can be associated with sudden groin pain as a potential fracture sign, acute loss of function, leg length discrepancy, and even local hematoma formation. In rare cases, even life-threatening bleeding can occur.^10,​25

Beside a pelvic anteroposterior (AP) view, CT diagnostics with multiplanar reconstructions are recommended. The main aim of radiographic diagnostics is to analyze the potential instability of the implant because secondary displacement of primary undisplaced fractures is common.

25.3.3 Nontraumatic Fractures

The presence of mobilization-triggered or stress-dependent pain, subjective instability, or axial malalignment can be signs of chronic component loosening. The knowledge of the prosthesis model is crucial in order to be sufficiently prepared for revision surgery. The type and number of already performed operations as well as a positive history of infection are also important for further planning. In late fractures, infection has to be ruled out (overview in²⁶).

Plain AP pelvic X-rays and a lateral view of the hip are followed by CT diagnostics, especially to address potential risk factors. CT analysis provides evidence of existing osteolysis and bone defects.

The main topic is to evaluate the stability of the implant. The extent of osteolysis can be an indirect sign of instability. Radiolucent lines, component migration, and position changes of the cup in comparison to former X-rays are further signs of component instability.²³

25.4 Classification

Several classifications of periacetabular/periprosthetic fractures are available. Classification of periacetabular fractures is influenced by the time of injury (intraoperative vs. postoperative) and the presence of bone defects.

Intraoperative fractures were first classified according to Challaghan et al,^16,​27 whereas postoperative fractures were classified according to Peterson and Lewallen.¹⁰ Postoperative fractures were subdivided into two categories depending on the stability of the cup component.¹⁰ The localization of the fracture line influences further treatment. Thus, analysis should focus on anterior column fractures (regularly with involvement of the pubic ramus), posterior column fractures (involvement of the ischium), medial wall involvement (quadrilateral surface), or cranial dome involvement.

Additionally, a defect classification aligns with the guidelines of the American Academy of Orthopedic Surgeons (AAOS) according to the D’Antonio classification,²⁸ because the quality of the periacetabular bone is essential for prognosis of further treatment. Correspondingly, if signs of cup loosening are present, a CT analysis of the osseous defect is mandatory.²¹

To create a sufficient management plan, Paprosky et al proposed a classification considering bone loss and stability of the cup component.²⁹ Davidson further simplified this classification to three main fracture types.¹⁷

Recently, the AO group recommended a more universal periprosthetic fracture classification (Unified Classification System [UCS]) with integration of anatomical aspects and location of the fracture.^30,​31

The UCS combines the AO/OTA fracture classification of the pelvis with special periprosthetic fracture parameters. Six different subgroups are integrated in this classification. These different classifications are summarized in ▶ Table 25.1, ▶ Table 25.2, ▶ Table 25.3, ▶ Table 25.4, ▶ Table 25.5, ▶ Table 25.6.

25.5 Treatment

Periprosthetic fractures mostly occur in elderly patients with multiple comorbidities. Open reduction and internal fixation (ORIF) is recommended even in osteoporosis, as immobility and insufficient weight bearing are common and can lead to further complications.³²

The primary goal of treatment in periprosthetic acetabular fractures is to achieve a stable cup component, avoidance of fracture propagation, maintenance of component position, maintenance of alignment, reconstruction of bone defects, and maintenance or restoration of the rotation center. Thus, the anterior and posterior column of the acetabulum must be stable enough to withstand motions of the implant. The bone implant interface is of major importance for long-term functioning of the cup.²⁴

Potential treatment concepts include conservative or operative treatment. In operatively treated patients, partial weight bearing is combined with either press-fit cup insertion with/without additional screws, implantation of Burch-Schneider antiprotrusion cups, reinforcement cage systems, socket cups, and/or osteosynthesis.

25.5.1 General Recommendations

Treatment depends on stability of the prosthesis and dislocation of the fracture.

Stable Prosthesis

In nondisplaced or minimally displaced acetabular fractures with a suggested stable cup component (type I according to Peterson and Lewalen¹⁰), conservative treatment is recommended.²⁷ If the cup seems to be stable but the pelvic ring has an unstable fracture, open reduction and plate osteosynthesis is favored. Depending on the fracture type, a posterior or anterior plate is performed.

In displaced fractures, even with a potentially stable cup, an extended cup loosening rate is expected due to possible increased shear forces.¹³

Unstable Prosthesis

In type II fractures according to Peterson and Lewallen,¹⁰ a combined cup revision with stable osteosynthesis is recommended.²¹ Due to the individual anatomy, reconstructions plates are favored.

An additional significant periacetabular bone defect has to be considered, especially when signs of ongoing cup loosening are present. CT analysis and classification of these defects is essential for optimal planning of the type of revision cup. In rare cases, custom-made implants are needed. Cementless cups show superior results than cemented cups.³³

Alternatively, revision systems (e.g., Burch-Schneider ring, supporting shells according to Müller or Ganz) are further options, but were associated with increased cranial migration in cases with posterior-cranial defects. Special fracture configurations can lead to special stabilization solutions.³⁴

25.5.2 Undisplaced Intraoperative Fractures

Intraoperative fractures are most often undisplaced and stable (type I according to Peterson and Lewalen¹⁰), and therefore conservative treatment is recommended.^{8,​27,​29}

In a series of 21 fractures, Haidukewych et al found 81% stable undisplaced fractures, which were sufficiently treated conservatively.¹⁴ Secondary displacement is possible,^7,​13 especially when the posterior column is involved. Two-thirds of posterior column fractures were associated with instability and secondary failure.⁷

Benozzo stated that these fractures show typically an incomplete vertical fracture line in the posterior column due to under-reaming or insufficient removal of osteophytes.⁸ Even in clinically stable cups, additional screw fixation is clearly recommended.^{8,​13,​24}

25.5.3 Displaced Intraoperative Fractures

In displaced fractures, even with a stable cup, an extended cup loosening rate is suspected due to possible increased shear forces.¹³ In unstable cups, a clear indication for osteosynthesis is stated. Benozzo proposed one or two posterior column plate(s) prior to definitive insertion of the chosen cup component. In the presence of bone deficits, the techniques of revision acetabular surgery are proposed.^8,​24 Surgery can be performed even in the lateral position.

25.5.4 Fractures Recognized Directly Postoperatively

The main problem in this group is that stability testing of the fracture and the implant is not possible.⁸ Thus, three potential treatment options are possible:

• 
  

  Minimal fracture displacement, stable cup, good positioned cup → conservative treatment, partial weight bearing, and short-term radiographical follow-up^6,​8

  
  
• 
  

  Minimal fracture displacement, malpositioned cup → posterior plate osteosynthesis, reimplantation of uncemented cup with additional screw fixation

  
  
• 
  

  Displaced fracture ± component malposition/migration → posterior plate osteosynthesis, bone stock reconstruction (▶ Fig. 25.1), if necessary, uncemented cup with additional screw fixation or insertion of a protrusion cage for additional superoinferior stabilization²⁹

  
  
  
  

  

  
  

  Fig. 25.1 Intraoperative posterior column fracture, postoperatively detected, with increasing displacement. Revision was indicated: a posterior plate osteosynthesis was performed with change of the cup, which was intraoperatively unstable.

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