Appearance of previously injured posterior cruciate ligaments on magnetic resonance imaging.

ABSTRACT

Background. The appearance of normal and injured cruciate ligaments on magnetic resonance imaging (MRI) has been well described in the literature. Few studies have reported on changes found on sequential imaging of injured posterior cruciate ligaments (PCLs) during the healing process, however.

Methods. Ten consecutive patients with isolated injuries to the PCL were evaluated with an initial clinical examination, x-rays, and MRI of the injured knee. Clinical examination and imaging studies were repeated at a minimum of 6 months after the initial injury. These studies were interpreted by radiologists blinded to the study design.

Results. Seven of the 10 patients were found to have PCL continuity on repeat MRI. One patient continued to demonstrate an abnormal appearance of the PCL on MRI, and 2 patients were lost to follow-up.

Conclusions. Despite the healed appearance of their PCLs on imaging studies, 6 of 7 patients continued to demonstrate laxity on clinical examination. All returned to their preinjury occupations and to recreational sports, however. Our results are consistent with previously reported data.

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THE magnetic resonance imaging (MRI) appearance of normal and injured cruciate ligaments has been well described in the literature. To date, however, only a few studies have reported on sequential imaging of injured posterior cruciate ligaments (PCLs) to see what, if any, changes in appearance occur during the early months of healing after injury to this ligament. (1-3)

MATERIALS AND METHODS

From August 1993 to June 1996, 10 consecutive patients seen by 1 physician with the diagnosis of isolated injury to the PCL were evaluated with an initial clinical examination, routine x-rays, and MRI of the injured knee (Table). A knee was considered to have an isolated PCL injury if the posterior drawer was <5 mm and decreased with internal rotation, and there was no varus, valgus, or anterior laxity.

Two of the patients could not be located for follow-up evaluations. The clinical examinations and imaging studies of the remaining 8 patients were repeated at a minimum of 6 months after the initial injury (range, 6 to 34 months). Magnetic resonance imaging was done using a GE 1.5 Tesla magnet (GE Medical Systems, Milwaukee, Wis) to obtain axial, coronal, and sagittal plane T1 and T2 imaging sequences. These studies were read as routine films by radiologists who were blinded to clinical data and study design.

RESULTS

On initial MRI evaluations done within several weeks of injury, all patients had abnormal signal in the PCL, with 5 patients' MRIs being read as consistent with a complete tear of the PCL (ie, there was loss of the normal low signal of the PCL, and no continuity of the PCL fibers was seen). Three patients' MRIs were interpreted by trained musculoskeletal radiologists evaluating the studies as showing loss of normal PCL low signal intensity "with a few of the ligament fibers possibly intact." On repeat imaging studies, PCL continuity was seen in 7 of the 8 patients studied; in fact, the PCLs were reported as normal by the musculoskeletal radiologists interpreting the follow-up MRI studies (Fig 1). On critical review of these films, however, all patients had an elongated appearance to their PCLs, rather than the arcuate configuration reported to be present in 88.2% of normal PCLs. (4) Only 1 patient showed persistent loss of good quality signal in the region of the PCL on repeat MRI (Fig 2).

Patients in this study were between 15 and 35 years of age (average age, 26 years) at the time of initial injury. The mechanism of injury varied in the 8 patients studied. Five patients were injured while participating in sports; 1 in a fall; 1 in a motor vehicle accident; and 1 in a bicycling accident. A hyperextension injury was described for 4 patients, and 3 had flexed knees when the injury occurred. One patient could not recall the position of her knee at the time of injury (Table).

All patients were initially treated after injury with a limited-motion brace for 2 to 6 weeks, followed by a period of rehabilitation ranging from 3 to 6 months. Only then were they permitted to return to pivotal activities.

At the time of follow-up, none of the patients complained of pain, locking, catching, swelling, or giving way of the knee. All had returned to their previous, occupations, as well as to recreational sports. The only patient who showed persistent loss of signal in her PCL on follow-up MRI did not return to soccer, the sport in which she had been injured, however, because she was afraid of reinjuring her knee. A 1+ posterior drawer with a definitive end point was noted on evaluation of 7 of the 8 knees, while stress test results were normal on 1 of the 8 knees.

DISCUSSION

The PCL is enveloped by a fold of synovium reflected from the posterior capsule. (5) The average PCL length is 38 mm and the average width is 13 mm. (6) It is narrowest at its midsection, arising from a broad site on the lateral wall of the medial femoral condyle and inserting into a recessed area measuring approximately 1 cm in the posterior tibia between the tibial plateaus. Its insertion extends distally for a few millimeters into the adjacent posterior cortex of the tibia. (5,7)

The meniscal femoral ligaments, which extend from the posterior horn of the lateral meniscus either anterior or posterior to the PCL, attach to the lateral aspect of the medial femoral condyle near the origin of the PCL. (7) These ligaments may aid in the stabilization of the tibia.

The blood supply of the PCL has been reported to be more plentiful than that of the anterior cruciate ligament. (8) The PCL blood supply originates primarily from the surrounding soft tissues, rather than from its osseous attachments. (8,9) The major contributing vessel is the middle genicular artery, which arises from the popliteal artery. (5,8,9) This artery also supplies the synovial sheath surrounding the PCL. The synovial sheath is itself a major contributor to the nourishment of the PCL. Other contributing vessels include the supreme genicular artery, the medial and lateral superior genicular arteries, the anterior and posterior tibial recurrent arteries, as well as the lateral femoral circumflex and recurrent fibular arteries.

Magnetic resonance imaging of knee ligaments has been well described. (4,10-12) Imaging of the PCL by this technique has a specificity and sensitivity estimated to be almost 100%. The image is considered positive for a tear when the normally large, homogeneously low signal-intensity band is noncontiguous, or the signal intensity is markedly increased with the ligament appearing swollen. Partial tears are seen as diffuse swelling of a portion of the ligament, with intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted images within this normally homogeneously, low signal-intensity structure. One must be careful to avoid interpreting increased signal intensity within the ligament due to field orientation on T2-weighted images as a tear, but recognize this "magic angle" phenomenon when it occurs.

The natural history of isolated PCL tears has not been well defined. Torg and Barton, (13) Dandy and Pursely, (14) Parolie and Bergfeld, (15) and Fowler, (1) have all reported satisfactory functional results in patients with this lesion who were treated nonoperatively, provided adequate strength of the quadriceps was achieved through rehabilitation and the degree of instability was less than 5 mm of posterior shift on stress testing.

Shelbourne, (16) in his review of 133 patients who sustained acute, isolated injuries to the PCL treated nonoperatively, found no correlation between objective and subjective knee function and the grade of laxity, as tested clinically. Of the 68 patients in the study who returned for follow-up evaluations, 63 had the same or less PCL laxity than they had immediately after injury; 5 patients had greater laxity. Regardless of laxity, Shelbourne (16) found that only one sixth of those injured could not return to the sport they had played before the injury.

Fowler et al (1) made a brief reference to findings seen on delayed (2 years after the injury) MRI studies done in 3 patients. The MRIs did show continuity of the PCL, even though these ligaments had previously been observed arthroscopically to have had complete midsubstance tears. No further comment was made by the authors regarding this finding.

Recently published studies by Tewes et al (3) and Shelbourne et al (2) further explore the MRI appearance of injured PCLs after an initial period of healing. In the study by Tewes et al, (14) 13 patients returned for evaluation between 5 months and 4 years after the injury. Seventy-seven percent showed continuity of the PCL on MRI, yet had persistent laxity on clinical examination. In the series of 23 knees reported by Shelbourne et al, (12) all knees with low-grade and mid-grade PCL injuries were found to have continuity of the injured PCL on follow-up MRIs at a mean time of 3.2 [+ or -] 1.3 years after their initial MRIs, as did 19 of the 22 knees with high-grade injuries. The authors concluded that, "these results demonstrated that most nonoperatively treated PCL injuries ...can heal with continuity." (2)

Our results are consistent with these studies, with MRI evaluations showing radiographic evidence of healing of this ligament after complete disruption. Perhaps it is the ligament's synovial tissue sheath, reflected from the posterior capsule that covers it on its medial, lateral, and anterior surfaces, that is able to support the injured ligament and supply it with a sufficient vascular supply to permit its revascularization and repair, as detected by MRI.

This is an interesting theory and consistent with Dandy and Pursely's indirect reference to natural healing of this ligament in their 1992 article, who stated that "...the exposure and dissection of such a ligament could jeopardize the chance of natural healing." (14) Further investigation, including arthroscopic evaluation of knees with PCL injuries with correlation to MRI findings at least 8 to 12 months after injury, would be extremely interesting and revealing.

The present study further substantiates earlier reports by Fowler et al, (1) Tewes et al, (3) and Shelbourne et al, (2) which note continuity and restoration of the low-signal intensity of injured PCLs on MRIs done more than 5 months after initial injury.

TABLE

Characteristics of Patients With Isolated Posterior Cruciate Ligament
Injuries

Patient   Age   Injury       Mechanism       Date of        Date of
  No.    (yrs)  Activity     of Injury     Initial MRI      F/U MRI

   1      28    Soccer     Hyperextension  June 1995     February 1996
   2      30    Fall       Hyperflexion    May 1995      February 1996
   3      35    Football   Hyperflexion    January 1994  January 1996
   4      30    Wrestling  Hyperextension  October 1994  December 1995
   5      21    MVA        Hyperflexion    October 1994  December 1995
   6      20    Baseball   Hyperextension  August 1993   February 1995
   7      35    Bicycling  Uncertain       March 1996    September 1996
   8      15    Soccer     Hyperextension  April 1996    January 1997

Patient      Status of PCL on
  No.           Repeat MRI

   1     Persistent loss of signal
   2     Normal signal
   3     Normal signal
   4     Normal signal
   5     Normal signal
   6     Normal signal
   7     Normal signal
   8     Normal signal

MRI = Magnetic resonance imaging, F/U = follow-up, PCL = posterior
cruciate ligament, MVA = motor vehicle accident.

References

(1.) Fowler P, Messieh S: Isolated posterior cruciate ligament injuries in athletes. Am J Sports Med 1987; 15:553-557

(2.) Shelbourne KD, Jennings RW, Vahey TN: Magnetic resonance imaging of posterior cruciate ligament injuries: assessment of healing. Am J Knee Surg 1999; 12:209-213

(3.) Tewes DP, Fritts HM, Fields RD, et al: Chronically injured posterior cruciate ligament: magnetic resonance imaging. Clin Orthop 1997; 335:224-232

(4.) Gross M, Grover J, Bassett L, et al: Magnetic resonance imaging of the posterior cruciate ligament: clinical use to improve diagnostic accuracy. Am J Sports Med 1992; 20:732-737

(5.) Fanelli G, Giannotti B, Edson C: The posterior cruciate ligament arthroscopic evaluation and treatment. Arthroscopy 1994; 10:673-688

(6.) Giris FG, Marshall JL, Almonagem ARS: The cruciate ligament of the knee joint. anatomical, functional, and experimental analysis. Clin Or hop 1975; 106:216-231

(7.) Van Dommelin B, Fowler P: Anatomy of the posterior cruciate ligament. Am J Sports Med 1989; 17:24-29

(8.) Clancy W: Anatomy and function of the posterior cruciate ligament. Clin Sports Med 1994; 13:509-518

(9.) Scapinelli R: Studies on the vasculature of the human knee joint. Acta Anat 1968; 70:305

(10.) Fischer F, Fox J, DelPizzo V, et al: Accuracy of diagnosis from magnetic resonance imaging of the knee. J Bone Joint Surg Am 1991; 73:2-10

(11.) Pope T: Magnetic resonance imaging and knee ligaments. J South Orthop Assoc 1996; 5:46-62

(12.) Sonin A, Fitzgerald F, Freedman H, et al: Posterior cruciate ligament injury: magnetic resonance imaging diagnosis and patterns of injury. Radiology 1994; 90:455-458

(13.) Torg JS, Barton TM: Natural history of the posterior cruciate deficient knee. Olin Orthop 1989; 246:208-216

(14.) Dandy D, Pursley R: The long term results of unrepaired tears of the posterior cruciate ligament. J Bone Joint Surg 1992; 64:92-94

(15.) Parolie J, Bergfeld J: Long-term results of nonoperative treatment of isolated posterior cruciate ligament injuries in the athlete. Am J Sports Med 1986; 14:35-38

(16.) Shelbourne KD, Davis TJ, Patel DV: The natural history of acute, isolated, nonoperatively treated posterior cruciate ligament injuries: a prospective study. Am J Sports Med 1999; 27:276-283

RELATED ARTICLE: KEY POINTS

* Patients presenting with mild to moderate clinical laxity of the posterior cruciate ligaments (POLs) after injury to the knee typically have an abnormal appearance of the PCL on magnetic resonance imaging (MRI).

* In patients who sustain an injury resulting in mild to moderate PCL laxity and undergo an initial period of protection, followed by an appropriate rehabilitation program, MRI 3 to 12 months after the injury typically shows a normal appearance of the PCL.

* Although patients who sustain PCL injuries resulting in mild to moderate ligament laxity do show radiographic restoration of the PCL at 6 to 12 months after surgery, most have residual clinical laxity.

* Unlike the anterior cruciate ligament, which obliquely traverses the middle of the knee joint, the PCL lies posteriorly in the joint, surrounded by a synovial reflection from the posterior capsule.

From the Peachtree Orthopaedic Clinic, Atlanta, Ga.

Reprint requests to Letha Y. Griffin, MD, PhD, Peachtree Orthopaedic Clinic, 2045 Peachtree Rd NE, Suite 700, Atlanta, GA 30309.