P Tibial Alignment

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Precise component alignment in both the anteroposterior and lateral planes is essential for proper implant function and longevity in total knee arthroplasty (TKA).

Inability to achieve proper alignment can generate eccen- tric implant loading resulting in early aseptic loosening and failure (Figure 12-1). In addition, correction of the mechanical axis of the lower extremity (Figure 12-2) to within 5 to 7 degrees of valgus has been shown to improve TKA implant longevity both biomechanically and clinically.^1–14

Both intramedullary and extramedullary alignment guide systems are used to correct deformity in TKA. Both systems are dependent on the degree to which each guide rod approximates the anatomic axes of the femur and tibia. Intramedullary alignment of the femur in TKA has been generally accepted as superior to extramedullary alignment.^15–22 The femoral shaft is difﬁcult to locate through a large, surrounding soft tissue envelope. Addi- tionally, femoral extramedullary alignment systems require estimation of the center of the femoral head.

Radiographic skin markers often can be used; however, bulky surgical drapes and obesity may present problems.

Alternatively, intraoperative ﬂuoroscopy or surgical navigation can be used to deﬁne the center of the femoral head.

On the tibial side, there is considerable debate as to whether intramedullary or extramedullary alignment is superior. Tibial intramedullary alignment devices are based on the assumption that the angle between the anatomical and the mechanical axis is not signifi- cantly different from zero in either the coronal or sagittal planes.^23–27This chapter seeks to define the indications and emphasize the contraindications for intramedullary alignment of the tibia in revision total knee arthroplasty. Furthermore, specific case examples are reviewed that illustrate the pitfalls of and alternatives

to intramedullary alignment of the tibia in total knee arthroplasty.

In our previous report,²⁸ 44 adult cadaveric tibiae without obvious clinical deformity were harvested. Using a stepped drill bit, the proximal medullary canal was entered anterior to the tibial attachment of the anterior cruciate ligament. The starting hole was oversized with a rasp and a long 8-mm diameter solid intramedullary ﬂuted guide rod was passed down the medullary canal until it was ﬁrmly engaged distally. The bone cut was made referencing off the intramedullary cutting jig.

Anteroposterior and lateral radiographs were taken and the anatomical, mechanical, and guide rod axes were assessed on each radiograph. The accuracy of the guide rod was assessed by measuring how closely the guide rod axis approximated the anatomic and mechanical axis in both the anteroposterior and lateral planes. The differ- ence between the anatomic axis and the guide rod axis was measured and deﬁned as the axis angle.

Observations obtained from this cadaveric study revealed that certain deformities and clinical situations would preclude the use of intramedullary alignment of the tibia in total knee arthroplasty. The clinician needs to be aware of the contraindications and alternatives to intramedullary alignment of the tibia in total knee arthroplasty.

RESULTS OF ANATOMIC STUDIES

Anatomic requirements for successful intramedullary alignment require a patent intramedullary canal for complete seating of the guide rod. In the cadaveric tibiae examined, analysis of the anteroposterior radiographs of all 44 specimens revealed the guide rod to be on average in 0.56 degrees of valgus (range 1.4 degrees varus to 2.8 129

Tibial Alignment

James V. Bono

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130 Revision Total Knee Arthroplasty

degrees extension) compared with the mechanical axis.

Analysis of the lateral radiographs of all 44 specimens revealed the guide rod to be in 0.2 degrees of extension (range 3.3 degrees ﬂexion to 2.5 degrees extension) compared with the mechanical axis.

The anteroposterior guide rod-mechanical axis angle was examined in 10% increments of guide rod insertion.

There was a tendency for this angle to increase as the insertion amount decreased, from 0.75 degrees at 90% to 100% insertion to 1.90 degrees at 40% to 50% insertion.

Maximum accuracy of the tibial intramedullary alignment guide rod required complete seating of the device to the level of the distal physeal scar (p < 0.05). The valgus tibiae, i.e., the tibia with a valgus bow, demonstrated an increased anteroposterior guide rod-mechanical axis angle as compared with the neutral or varus tibiae. Fur- thermore, the intramedullary guide was more accurate in reproducing the mechanical axis in the non-valgus tibiae (p< 0.05). This ﬁnding suggests that the valgus tibia may be a relative contraindication to relying exclusively on intramedullary alignment.

In addition to the ﬁndings described previously, other clinical situations can prohibit the use of intramedullary alignment in total knee arthroplasty. Any situation that blocks the passage of a straight guide rod would disallow the use of intramedullary alignment. Both anatomic abnormalities and retained implants can result in mechanical obstruction of the intramedullary canal (Figures 12-3A, B and 12-4A, B).

OBSERVATIONS IN REVISION TOTAL KNEE ARTHROPLASTY

The incidence of revision TKA is increasing, largely due to the increased number of primary procedures per- formed annually. The leading indications for revision TKA include reimplantation after infection and aseptic loosening. Bone stock loss is invariably encountered at revision resulting from mechanical collapse of bone, oste- olysis, or a result of aggressive debridement in the setting of post-septic reimplantation. The use of intramedullary stems in this setting is advisable due to the compromised bony platform of the tibial plateau, as well as to offset the stresses transmitted to the bone, which accompany the use of constrained and semi-constrained revision components.

Intramedullary extension stems may be used both with and without cement and are discussed further in the following chapter. Cementless fixation is typically achieved by intimate contact of an uncoated, fluted extension stem within the intramedullary canal of the tibia and femur. The intramedullary canal is prepared with rigid axial reamers to match the diameter of the selected intramedullary extension stem. The intramedullary extension stem is assumed to replicate the intramedullary axis of the femur or tibia. As a result, component position is dictated by the use of an intramedullary extension stem. If a cementless extension stem is selected, greater stability of the intramedullary extension stem occurs with circumfer- ential filling of the stem within the intramedullary canal.

FIGURE 12-1. Massively obese 70-year-old woman with early mechanical failure following TKA. Varus alignment of the tibial component contributed to mechanical overloading of the medial compartment.

FIGURE 12-2. Proper alignment of the femoral and tibial com- ponent allows even distribution of stress over the medial and lateral compartment.

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A B

FIGURE 12-3. (A and B) AP and lateral views of the tibia depict a well-healed fracture of the tibial diaphysis, which would block the passage of an intramedullary guide rod into the tibia.

A B

FIGURE 12-4. (A and B) Nonanatomic alignment of the tibial diaphysis precludes the use of intramedullary alignment.

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Intramedullary extension stems may be used in two distinct manners, based on surgeon preference. First, if the surgeon elects to emphasize stability of the stem within the canal based on a line to line ﬁt, the component

position will by necessity be dictated by the intramedullary stem, and may not result in symmetric coverage by the underlying bone (Figure 12-5). If, however, the surgeon prefers symmetric positioning of the component, the diameter of the intramedullary extension stem may have to be compromised, to shift the component from the intramedullary axis of the tibia or femur (Figure 12-6A, B). If this is done, the stability of the cementless stem within the canal will suffer. Stability may be recovered by cementing the stem within the canal, acknowledging an asymmetric cement mantle.

If an intramedullary extension stem is used, component position will be dictated by the position of the intramedullary rod. In a previous study,²⁹ we sought to determine whether the use of a press-ﬁt, canal-ﬁlling, cementless intramedullary extension stem in revision TKA resulted in asymmetric placement of the tibial component.

RESULTS OF RADIOGRAPHIC DATA

Radiographs of 24 patients undergoing revision total knee arthroplasty with a stemmed tibial component were reviewed. The same modular revision implant system was in each case. There were 14 male and 10 female subjects, with an average age of 66.7 years (range, 37 to 93).

A B

FIGURE 12-5. Following revision TKA using a press-ﬁt intramedullary tibial stem, the tibial component is noted to overhang medially, leaving the lateral plateau uncovered. The position of the tibial component is dictated by the placement of the stem and does not always result in symmetric coverage of the tibial plateau.

FIGURE 12-6. (A) An attempt to place the tibial component symmetrically on the tibial plateau results in non-anatomic placement of the tibial stem, illustrating the conﬂict between the intramedullary axis of the tibia and the anatomy of the tibial plateau. (B) A custom-made tibial component with an offset tibial stem allows for axial alignment of the stem with anatomic coverage of the tibial plateau.

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Intramedullary tibial stem extensions were used in each case, with an average diameter of 14.9mm (range, 10 to 20mm) and an average length of 68.5mm (range, 30 to 115mm). Augmentation wedges were required in 5 patients, with two 10 degree full medial wedges, one 15 degree full medial wedge, one 15 degree half-medial wedge, and one 10 degree half-lateral wedge. Measure- ments of tibial component medial, lateral, anterior, and posterior displacement were made and corrected for magniﬁcation.

The tibial component was noted to be eccentrically positioned on the tibial plateau in 24 of 24 patients, with medial placement noted in 20, lateral in 3, posterior in 17, and anterior in 3. Medial tibial component overhang was most common (46%), averaging 2.5mm (range, 1.7 to 4.3mm). Of the 11 patients with medial component overhang, the lateral aspect of the tibial plateau was noted to be uncovered by an average of 5.4mm (range, 1.8 to 9.9mm) in 8 patients.

IMPLICATIONS FOR REVISION TOTAL KNEE ARTHROPLASTY

Medial eccentricity of the tibial component was found to be the most common problem (20 of 24) encountered when intramedullary extension stems were used in revi-

sion TKA,²⁹resulting in medial overhang in 11 of 24 cases despite downsizing of the tibial component. Posterior placement of the tibial component was similarly noted in 17 of 24 cases. This is the result of altered anatomy due to loss of proximal tibial bone stock and the restriction placed on tibial component positioning by the intramedullary stem. This ﬁnding suggests that an allowance for lateral and anterior offset be incorporated into tibial component design when used with an intramedullary stem extension (Figures 12-7 and 12-8).

Therefore, if an intramedullary extension stem is used, component position will be dictated by the position of the intramedullary rod. Asymmetric placement of the component typically results. A component, which would be of appropriate size, is found to overhang on one side and be uncovered on the other. This typically requires downsizing of the component to remedy the overhang, which accentuates the amount of bone uncovered by prosthetic component. The results of this study con- firmed our belief that the use of a canal filling cementless, press-fit intramedullary extension stem creates asymmetric positioning of the tibial component.

DISCUSSION

Appropriate orientation of prosthetic components is crucial for arthroplasty survival. Postoperative alignment of the lower extremity has a direct effect on the durabil- ity of the implant. Signiﬁcant varus or valgus malalignment may predispose the tibial component to early loosening.

FIGURE 12-7. A modular offset tibial stem is used to shift the tibial component laterally and posteriorly to allow symmetric coverage of the tibial plateau. The press-ﬁt tibial stem is centered within the diaphysis and ﬁlls the canal.

FIGURE 12-8. An offset adapter (Stryker, Allendale, NJ) is avail- able in 4, 6, and 8mm increments and is used to shift the tibial component (360 degrees) about the intramedullary axis, which is deﬁned by the intramedullary extension stem.

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FIGURE 12-9. Previous fracture has distorted the tibial metaph- ysis, which must be recognized in order to achieve proper alignment and ﬁxation.

FIGURE 12-10. Posttraumatic arthritis following ORIF of a tibial plateau fracture. The tibial metaphysis has been distorted.

Hardware is removed before TKA.

FIGURE 12-11. A lateral tibial plateau fracture with bone loss.

Hardware is removed before TKA.

FIGURE 12-12. A 2-stage reconstruction is planned. The ﬁrst stage consists of hardware removal with simultaneous creation of fasciocutaneous ﬂaps, which tests the integrity of the soft tissues before implantation.

Anatomic deformity can result from previous fracture (Figure 12-9), sepsis, or metabolic bone disease (e.g., Paget’s disease). Implant barriers to intramedullary alignment occur after fracture ﬁxation (Figures 12-10 through

12-12), broken retained hardware, or below a femoral component in total hip arthroplasty.

Whether an intramedullary or extramedullary alignment guide is used, accurate reproduction of bony cuts is

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a prerequisite for successful arthroplasty. Either guide system relies on the similarity between the anatomic and mechanical axes. Our previously reported cadaveric tibiae data conﬁrm this assumption; the anatomic axis approached the mechanical axis to within 1° on average in both the anteroposterior and lateral planes.²⁸

For the tibia, many surgeons prefer extramedullary alignment, using bony landmarks about the ankle as ref- erence points. Because the center of the talus is slightly medial to the midpoint between the malleoli, the surgeon must estimate the center of the talus based on these bony landmarks, which may be obscured by soft tissue excess, bony abnormalities, or bulky surgical drapes. Even if surgical navigation systems are employed, alignment is still based on where the surgeon estimates the center of the talus to be located.

Some authors have suggested that for the tibia, intramedullary alignment is more accurate and repro- ducible than extramedullary alignment and allows consistent and accurate long bone cuts. Our cadaveric tibiae data conﬁrm the reliability of intramedullary alignment in assessing the anatomic axis in total knee arthroplasty.

However, when passage of the intramedullary guide rod is prevented from complete seating to the distal tibial physeal scar, the reliability of this technique in assessing the anatomic axis of the tibia is impaired. Simmons et al.

were unable to template a long tibial intramedullary guide rod from a central entry point in 42% of cases. In addition, they were able to achieve a 90 degree cut to the long axis of the tibia in 30 of 35 knees (85.7%) when complete seating of the guide rod was achieved and only in 2 of 25 knees (8%) when the long tibial intramedullary guide was incompletely seated.²⁴ Our data demonstrate that when penetration of the guide rod was incomplete, the resultant malalignment corresponded inversely with the depth of insertion. In cases in which penetration of the guide rod was complete (>80%), the accuracy of the intramedullary alignment system increased (p < 0.05) to within 1 degree in both the anteroposterior and lateral planes.

Angular deformities in the tibia can interfere with the use of intramedullary devices and prevent passage of the guide rod. Simmons et al. suggested that intramedullary alignment is less predictable in the valgus knee and may lead to malalignment. Our data support the decreased accuracy of tibial intramedullary alignment in valgus versus neutral and varus tibiae (p < 0.05). Therefore, valgus deformity of the tibia may be a contraindication to absolute reliance on intramedullary alignment.

In addition to a valgus bow of the tibia, anatomic bony deformity may be a contraindication to the use of intramedullary alignment when performing total knee arthroplasty. Previous fracture, osteotomy, sepsis, or

metabolic bone disease, such as osteopetrosis or Paget’s disease, can result in a long bone deformity of the tibia that precludes the use of intramedullary alignment guides. Furthermore, retained hardware after fracture ﬁx- ation or intramedullary cement/hardware after total knee arthroplasty act as barriers to intramedullary alignment.

Careful preoperative planning with standing long leg radiographs will identify the patient at risk for incomplete passage of an intramedullary alignment guide rod and should be obtained in all TKA candidates in whom an intramedullary alignment system is considered.

CONCLUSION

There is considerable debate whether intramedullary or extramedullary tibial alignment provides a more accurate reproduction of the mechanical axis of the affected limb.2,17,18,23,28In the absence of severe bowing of the tibia, which precludes complete seating of the guide rod, intramedullary tibial alignment is reproducibly accurate and consistent to within 1 degree in the varus-valgus and ﬂexion-extension planes. Maximum accuracy of tibial intramedullary alignment requires complete seating of the device to the distal tibial physeal scar (p < 0.05) and is best suited for the nonvalgus tibiae (p < 0.05).

Theoretical disadvantages of intramedullary alignment in TKA include the increased risk of fat embolization and medullary bone loss with guide rod passage to the tibia. A reduction in guide rod diameter from 8 to 6mm, in conjunction with lavage and suction of the intramedullary canal, can help decrease the potential for fat embolization during insertion of intramedullary alignment devices. Anatomic angular deformity resulting from previous fracture, osteotomy, sepsis, or metabolic bone disease may represent additional contraindications to intramedullary alignment use. Furthermore, mechanical obstruction resulting from retained hardware after fracture ﬁxation, osteotomy, or intramedullary cement/

hardware after total knee arthroplasty may preclude the use of an intramedullary guide rod. Careful preoperative planning identiﬁes the patient at risk for incomplete intramedullary guide rod passage. In these patients, the use of extramedullary alignment and intraoperative radiographs maximizes accuracy of tibial component position and improves implant longevity.

In revision TKA, alignment is equally critical. Our data have shown that the intramedullary axis of the tibia does not bisect the tibial plateau.²⁹Therefore, if a cementless intramedullary extension stem is used, tibial component position will be dictated by the position of the stem.

In the majority of cases, this results in asymmetric position of the tibial component with respect to the tibial

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plateau. This creates the potential for component overhang and diminished support. The use of offset cementless intramedullary extension stems is recommended to address these shortcomings. An asymmetric stem reduces the potential for component overhang while reclaiming areas of uncovered bone for component coverage. In most cases, the need to downsize components is eliminated, allowing a larger component to be used; this allows for an increase in surface area for component support and ﬁxa- tion. The results of this study support the use of an offset stem, which allows for both anteroposterior and medio- lateral translation to maximize bony contact between the tibial component and host bone.

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