The tibial insert of a modular tibial component is attached to the tibial baseplate with a mechanical locking mechanism. A number of different locking mechanisms have been used successfully in TKR including those with peripheral and/or centrally located mechanisms which capture the tibial insert.

Dissociation of the tibial insert from the tibial baseplate usually results after TKR if the insert is incompletely seated at the initial surgery [11, 13]. Cement or soft tissue which becomes interposed between the insert and baseplate can prevent full seating of the insert and result in dissociation. This may be more of a risk with minimally invasive surgery in which visualization of the baseplate locking mechanism is more limited [11]. Displacement forces on the tibial insert are generally directed anteriorly, but can occur posteriorly [4, 17]. Dissociation of the insert is apparent on a lateral radiograph. Treatment should consist of revision to carefully inspect the integrity of the baseplate locking mechanism and revision of the insert provided the locking mechanism is intact.

Insert dissociation can also result from mechanical failure of the UHMWPE insert at the insert locking mechanism [17]. This mode of failure has occurred more frequently in modular acetabular total hip components, including those made of highly cross-linked UHMWPE [7]. Constrained components which are used to treat ligament insufficiency have higher stresses on the locking mechanism than unconstrained components with intact ligaments. Constrained inserts which rely on a central screw to secure the insert to baseplate may dissociate if failure of the screw fixation occurs [4, 16]. Fully constrained rotating hinge mechanisms can also dissociate if sufficient soft tissue laxity is present to allow the rotating hinge stem to be completely distracted from the tibial component (Fig. 51.2).

Femoral fracture at the tip of a revision TKR femoral or tibial component diaphyseal stem can be a difficult problem to manage. Treatment options include revision to a longer stemmed component and/or cable plating or strut allografting (Fig. 51.3).

Periprosthetic fracture may occur from minor trauma if a stress riser is present at or near the stem tip. This may result from perforations in the cortex from prior hardware or reaming the intramedullary (IM) canal (Fig. 51.4). A stress riser also occurs if the proximal femoral stem tip is in proximity to the distal stem tip of a THR. Depending on the stem lengths, there may be a short distance between the two stems which results in an area between the stems at risk of fracture. Fracture between the distal tip of a THR stem and proximal tip of a TKR stem can be associated with considerable morbidity [10]. Treatment with plating allows retention of both the THR and TKR. However, if nonunion develops, then revision to a total femur is likely to be required. Patients with osteopenic bone, poor balance or cognitive dysfunction, and IM femoral THR and TKR stems in proximity to one another would be expected to have a risk of falling and developing a periprosthetic fracture between the two stems. Patients at risk of interprosthetic femoral fracture between THR and TKR stems may benefit from prophylactic femoral plating at the time of revision TKR (Fig. 51.5).

Patella infera can occur from contracture of the patellar ligament following trauma or surgery and may develop in association with arthrofibrosis [14]. Arthrofibrosis can develop from a number of causes and results in formation of dense intra-articular scar formation [19, 21]. The combination of patella baja and arthrofibrosis is difficult to treat since patellar ligament contracture shortens the extensor mechanism contributing to limited knee flexion.